We've got just over two weeks until the Season 4 final begins. The villain will be somepony Celestia knows from the past, strong enough to defeat Discord, and Twilight will have to use a Spirit Bomb with energy provided by the other princesses to defeat him/her. That, or the villain will taste the rainbow power! So, there are many theories on who the villain is. I think it's not going to be Discord based on the second summary, but who do you think it'll be?
(new question posted after three people voted)
Here's my views on each theory:
1.) Nightmare Moon's ghost - They did say "from the past" but how far? How is she suddenly a bigger threat? I suppose the Mane 6 don't have the Elements now, plus maybe when they struck her down she became more powerful... (Debunked - It is a male villain)
2.) Queen Chrysalis - I doubt they'd have her return since they're afraid of ruining the character or either confirming/denying the comics (to please those who take either view), and if they don't show any regard for the comics, all the more reason they intend her dead from that fall. Still, Meghan is dodging questions about her (unlike Sombra) so I could somehow see her draining Crystal Heart power and becoming the big enough threat. The episode does look like it's set at the Crystal Empire (Debunked)
3.) King Sombra's ghost - Unlikely. Besides, some think he was already a ghost when Cadance and the crystal ponies destroyed him. Meghan said he was KilledOffForReal.
4.) Tirek - Although he died in G1, G4 is a separate continuity (like the Batman series, Jack Nicholson's Joker died in the Tim Burton series but the Dark Knight trilogy has a Joker of its own with a different origin and fate) since Spike has entirely different origins, and he and Applejack live a thousand years later than Firefly's time (while all three fought Tirek together in G1). Also maybe mention of Firefly 1000 years ago (which seems to be the magic number for villains) in Testing is foreshadowing Tirek. This would make "Discord vs. Tirek" canon, and maybe he was the one behind Nightmare Moon. (Confirmed, if the new promo is legit)
5.) Other G1 villain - Others have suggested villains like the Smooze (who I only found out about recently and he dies just like Tirek and Lavan), or Grogar (who gets banished in G1), or other G1 villains. The only G1 villains I'm familiar with are Tirek and Lavan. (And since the villain is a magic-stealer Lavan is extremely likely)
6.) Pony of Shadows - This character does seem quite mysterious. Although if it is a manifestation of Nightmare Moon magic, how could it be this powerful? Unless it has a way of gaining power and is doing do for a thousand years. Also since this will coincide with Twilight getting her kingdom, and Celestia and Luna's old castle seems haunted by the Pony of Shadows, it's possible. Sneak peak at final antagonist! (Debunked - This villain was in Tartarus until Twilight's Kingdom while the Pony of Shadows is in the old castle)
7.) Starswirl the Bearded - This may overlap with the Pony of Shadows, who I think is Starswirl given the shadow of the hat looking similar to his. Some think Starswirl will be corrupted by Nightmare Moon magic, others think he is evil to begin with since Celestia said he didn't know friendship the way Twilight does. This would probably retcon the Reflections arc of the comics, but still. There have been numerous references to him throughout the series and he is a powerful unicorn so it would make sense.
8.) Applejack - Get real, if she's the villain this episode then I'm a monkey's uncle. (Debunked, as if it wasn't already obvious she's not even evil)
9.) Princess Celestia - Y'all need to stop with the "Celestia turns out to be evil" theories (Debunked)
10.) Entirely new villain - While it seemed to me they were out of new villains in Princess Twilight Sparkle given the quality of the Plunderseeds, they did create two new interesting villains in The Mane-iac and the chimera, so maybe they've still got it. The whole "new threat" line implies it will be a new villain instead of an old one. Which still means it could be Starswirl, Tirek, or Pony of Shadows, but maybe there will be an entirely new villain. Have they done an Emperor Palpatine villain yet? They've done Sauron (Sombra), and Voldemort (Chrysalis), in a sense... maybe this new villain is an evil Emperor?