OK, so there's lots of talk about Season 4 coming up, and whether it'll be the last season of the show? I think it will, for the following reasons.
They can't seem to come up with any more villains, or at least any to top Sombra. Each villain has been darker than the last except for the fifth.
Nightmare Moon - Standard "eternal darkness" villain. Doesn't seem too dark, just standard, even though she a.) tries to kill the Mane 6 by sending them falling to their deaths, and b.) is hinted by Lauren Faust (and also by the comics, to those who take them as canon) to be the embodiment of Luna's jealousy manifested into a form of its own to demonically possess her, but this is not mentioned much in the show and the only hint to this is pieces of Nightmare Moon laying around Luna when she is restored, as well as Luna saying she missed Celestia "so much" when Nightmare Moon would have felt only vengeance for imprisoning her. But still the least evil of the first four since her goal was to bring eternal darkness, and become the sole ruler of Equestria, and would have been a tyrannical ruler.
Discord - He doesn't seem as threatening as Nightmare Moon, and seems more like a mischeivous goofball and fun to have around with his magic tricks, but underneath that is something darker, as he is said to make life miserable for ponies and also corrupts the Mane 6 to turn against each other. Nevertheless, he is not pure evil, as Fluttershy reforms him.
Queen Chrysalis - The first Knight of Cerebus. As if putting Shining Armor under a spell to weaken him and planning to take over and destroy Canterlot wasn't enough, she has Celestia, Shining Armor, and Twilight's friends turning against Twilight of their own accord in the single most depressing moment of the series, then traps Twilight, along with Cadance, in forgotten caves intending them to starve to death, has her own song with one particularly sinister verse (I'm sure we all know the one), is more menacing in appearance and voice than Nightmare Moon, and although this is not clearly elaborated on, there are theories that when she traps Celestia in a cocoon, she intends to feed her to changelings.
King Sombra - The second Knight of Cerebus, even darker and more menacing than Chrysalis. His evil laugh sounds like Andross from Star Fox 64. He enslaved an entire empire (and is presumed to have killed its previous ruler to take it over), the mere thought of him brings pain and fear to the crystal ponies, he is known to subject ponies to their worst fears, and he tries to impale Spike with a dark crystal. Implied to be the most powerful and intelligent of the main villains, seeing how powerful he is already in his weakened shadow state, imagine what power he possesses as a unicorn, and while he is a pony of few words, his intelligence is clear with all the traps he set to protect the Crystal Heart, which would have defeated Twilight on more than one occasion if Spike wasn't there. Unlike the previous three, he doesn't screw around with his enemies, anypony that could be seen as a threat to him he'll kill them at the first opportunity, as he would have done to the Mane 6, Shining Armor was lucky to get out of that fight with only his horn cursed. His motives are unclear which makes him more mysterious, and he is obviously pure evil as the Crystal Heart completely destroyed him with its magic of love and light.
Sunset Shimmer - Kind of a mix of Trixie, Gilda, Diamond Tiara, and Flim and Flam. Basically, she's just a jerk, just one step above the others with her power-hungry ambitions, as it's clear she wants to use Twilight's crown to take over Equestria. While she is shown to be cruel, drives friendships apart, and tries to sabotage Twilight to keep her from winning, she is also shown not to be violent, as she refuses to kill Spike and lets him go before threatening to destroy the mirror. All in all, the least evil of the five main villains, when you'd expect more from a full movie (think Pokemon movies). Although she DOES get more evil when in demon form, and there is possibility her demon form is the manifestation of her cruelty as Nightmare Moon is Luna's jealousy, but this is debatable because her name does not change and while she is now pure evil, she still talks as Sunset Shimmer. So it seems they couldn't come up with anything to top Sombra. Hay, they're both unicorns, and one looks like any regular unicorn, the other is really elaborate with fangs, a demonic-looking horn, etc.
And Sunset Shimmer's inferiority to her predecessors is only the beginning. Season 4's two-part premier? Will bring back Nightmare Moon, most likely in a flashback that Twilight will be sent to experience from Celestia's point of view. So while there is possibility that it could even be a set of flashbacks dealing not only with Celestia's fights with Nightmare Moon, but Discord and Sombra as well, and could give more insight as to the characters of Nightmare Moon and Sombra, it doesn't seem there will be a new villain.
So they're running out of ideas for villains. They can't make any to top Sombra (although some say Sunset Shimmer's demon form does because she tries to kill Twilight). Maybe any that would top him would be too dark for an all-audiences show (who's the darkest villain we've seen in Disney movies, btw?). But the creators have hinted at a BigBad, a villain behind Nightmare Moon and Discord, maybe connections to Chrysalis and Sombra as well. Although his/her motives could be unknown since all four are acting for their own reasons and ambitions for power, maybe he/she just wants Equestria destroyed or plunged into darkness but prefers to stay in the shadows.
Anyway, if there is a BigBad, it'll probably be at the end of the series, and they probably have plans for him/her as well, and such a villain is most likely to be darker and eviler than Sombra. There's rumors it might be a G4 version of Tirek, but there's also talk that Hasbro "lost the rights to him when they stopped doing G1" (which doesn't make sense, who'd they lose it to? Are the Hasbro staff members that created G1 gonna rise from the grave to sue them if they use him?) And since they have no ideas for other villains, and used a flashback as a placeholder for Season 4's premier, it's likely to be either in a two-part season finale for 4, or a movie set after.
EDIT: Season 4's premier will feature a new villain. And apparently one as dark as Chrysalis at that, maybe even Sombra, if he/she plans to "destroy everything". Maybe I underestimated the writers when given the chance to use their full creativity.
Season 3 reception
Lauren Faust left the show after Season 2. Without her, all they had to go on to keep up MLP's usual "appealing to bronies" style was what they thought we wanted them to do, which they assumed was anything that was commonly depicted in fanfics or fanart. They couldn't be more wrong. A lot of it we didn't want to happen in canon. There is rumors, however, that Hasbro interfered with the writers' creativity for whatever reasons. And Season 3 was an utter failure, even if I liked the episodes The Crystal Empire, Keep Calm and Flutter On, and Magical Mystery Cure, in which I am definitely in the minority (the only episodes the majority seemed to like were Magic Duel, Sleepless in Ponyville, and Wonderbolts Academy), it received poor reception among the majority of bronies (and Lauren Faust herself), and probably half left the fandom after the controversial season finale where Twilight becomes an alicorn. Twilight has wings and is a princess now, she's fourth alicorn, and in the confirmed Season 4, which has been planned since before Season 3's release, she'll be an alicorn the whole way through, upsetting the balance of the Mane 6 (two unicorns, two earth ponies, two pegasi) and changing the series from what we knew and loved. I'm speaking for the majority and the haters, but not for myself because I am looking forward to more of our Twilight Princess in Season 4, and think it SHOULD be generating jokes connecting her to the Zelda game.
They had intended Season for for quite a while, but Season 3's reception was so bad that it probably cancelled any potential plans for a Season 5. Like Saw was originally intended for eight films, but Saw VI didn't get as good reception so they ended it at VII by merging the intended scripts for VII and VIII. Besides, people probably don't want more than one season of Twilight as an alicorn.
On that note, they've got about half of the brony audience still remaining, maybe less, there's even rumors that the amount of younger viewers (which Hasbro initially intended MLP for, it was Lauren Faust who made it what it is), exceed that of bronies and pegasisters now due to those who left the fandom because of Alicorn Twilight. Between the reduced audience and having monkeys on their marketing staff (as I can't think of any other reason behind the poor marketing of Equestria Girls), Hasbro won't be making as much off MLP as they did for the first two seasons.
Running out of ideas
It seems like they're running out of ideas in Season 3 so it's amazing they're even going through with Season 4. However I felt about King Sombra and The Crystal Empire, it cannot be denied that King Sombra still doesn't do much and much of the episode revolved around lighter scenes of the crystal ponies. While it is true that King Sombra didn't have a chance to do much, maybe the real reason was they couldn't think of what to do with him. Although they DID give him a spectacular (and, in a first for the series, official) death scene, many felt he was defeated too easily. Also, King Sombra was the "Sauron" of MLP, which they needed to use at some point seeing how much of MLP was inspired by Lord of the Rings. So they've already played the Sauron card.
Too Many Pinkie Pies does the "multiple clones" thing, One Bad Apple is another episode about bullying, which they previously had to a somewhat lesser scale with Gilda in Season 1, Sleepless in Ponyville was about nightmares and appearances by Luna establishing her as entering ponies' dreams, Wonderbolts Academy finally took a step for Rainbow Dash towards becoming one of the Wonderbolts, Apple Family Reunion, somehow didn't seem as interesting to many, myself included, except for the notablility of Applejack getting her own song and her parents confirmed to be dead. Spike At Your Service is a typical "character saves another character's life and they become their servant" thing, and to make Applejack try to make Spike stop being her servant, they made him majorly out of character by screwing up almost every job he does (seriously, he's not like that with Twilight). Keep Calm and Flutter on saw the return of Discord, bringing back an old villain and reforming him. Just for Sidekicks, probably one of the least interesting episodes of the series, aside from taking place at the same time as Games Ponies Play, which plays the "mistake one pony for another throughout the episode" card. I'm not saying all of them weren't good episodes, though, Discord return was cool and of course we all love Luna appearances, just saying since they used some of these ideas once, doing it again would be repetitive. Magical Mystery Cure, while being one of my favorite episodes of the season, people complained it seemed "rushed", and Twilight becoming an alicorn, though I don't think anypony's used the term yet, was essentially "jumping the shark".
The one episode that didn't make the season seem like they were running out of ideas was Magic Duel, which despite seeing the return of Trixie, makes her evil by putting the Alicorn Amulet on and corrupting her, and that whole elaborate trick in the magic duel to trick Trixie into taking it off was amazing.
So, after playing all those cards in Season 3, exactly what can they think of for episodes of Season 4 that they haven't done already? The only thing I can think of, aside from the flashback sequences and revealing the BigBad, is an episode where the Mane 6 switch bodies (which they might think would be too much like what happened in Magical Mystery Cure with the cutie marks). Oh, and return and redemption by Lightning Dust, that would be cool. (Or would they consider it too repetitive of Trixie?) But since they haven't turned aside from Season 4, they'll have to stretch REALLY far and come up with anything, ANYTHING, they can. They might even go as far as:
- Three separate episodes for each of the Cutie Mark Crusaders getting their cutie marks. Which in any other situation I'd say would be impossible because they are so oblivious to what's right in front of them
- Make the other Mane 5 alicorns. (Maybe even an episode each focusing on their transformations, kinda like Winx Club does with Charmix and Enchantix, or Sailor Moon whenever Sailor Scouts get new attacks, or Digimon whenever they digivolve to Ultimate)
- Making Discord a villain again, thus disregarding what Twilight said about Discord not wanting to lose Fluttershy as a friend, and Celestia "sensing a big change in Discord"
- Bringing Queen Chrysalis back, despite her presumed death in the show and, to those who take comics as canon, her horn being broken in two. In which case they're sure to either kill her or reform her.
- An episode to officially canonize the comics and take their style
- Bringing King Sombra back as a ghost.
- Bringing Gilda back
- Episodes revolving around background ponies
And Season 3 didn't have a lot of songs, or notes to Celestia either. There were three songs in The Crystal Empire, one in Babs Seed, and one, I think, in Apple Family Reunion, none of the other episodes had any except for the musical episode, Magical Mystery Cure.
Also of note, I am one of the least demanding bronies in the fandom, yet I'm disappointed by Season 4 already, because my one demand was that they bring in a new villain as dark as Chrysalis for the Season 4 premier and destroy him/her as spectacularly as Sombra. And we won't be getting that, in favor of a flashback sequence. Now the season finale might make up for that though. I know this is minor, especially since it could show the Sombra battle as well and give him more screentime and lines, but still.
The only hope for Season 4 is that they said it would be the best season, and that it would keep the bronies happy. I'll believe that when I see it, I'm sure I will find it to be good but I'm not so sure about the others, a lot of them seem quite picky and demanding these days (I know there are many who aren't, but still). Also, they mentioned a song in Season 4 to be catchier than the cafeteria song in Equestria Girls, which is why I haven't said they used up all their songs in MMC. Also, there's rumors that Hasbro realized their style for Season 3 wasn't the way to go and won't be interfering with the creativity of the writers, many of which were there through the first two seasons. In which case, let Season 4 be the grand finale for the series and end MLP on a positive note. They'll be pouring all their effort into this season to make it top Season 2.