So, the last Mane 6 teaser for Season 5 was just released (Rarity) and seemed to show another scene of Season 5 where Rarity has her cutie mark replaced with a "=" symbol. This is the same symbol found in the Rainbow Dash trailer that appeared earlier this month. The two scenes are probably from Season 5. However, if we're going to get to the bottom of this, we'll have to go back about 5 months to the Season 5 animatic.
The Season 5 Animatic
The Season 5 Animatic was shown at the SDCC 2014 Panel this year. The entire animatic was basically a storyboard of the first episode of Season 5. In it, the Mane 6 are sitting in Twilight's new castle (Season 4 finale) and find a map of Equestria on the table that sits in the center of the main room, with their thrones encircling it. In the map there appears to be an unknown town (located in an unknown area to us so far). Throughout the animatic, the Mane 6 find out that the town is inhabited by normal ponies who all have the same exact cutie mark. - an equal sign. Each pony in the town is shown to also have an uncanny smile (almost as if they're forced to keep a smile).
Rainbow Dash Teaser Trailer
The Rainbow Dash teaser trailer shows about three seconds of Season 5. In these three seconds, we see Rainbow Dash with an equal sign cutie mark while also struggling to fly. The background of the scene (which looks similar to that of the animatic) may suggest that this scene takes place in or near the town shown in the animatic. Anyway, this may suggest that Rainbow Dash's new equal sign cutie mark may have an effect on her flying capabilities.
Rarity Teaser Trailer
Rarity's teaser trailer also shows about three seconds of Seaosn 5. In these three seconds, Rarity is also seen with an equal sign cutie mark while also sobbing. It is unknown why she is upset, but it is probably because of her new cutie mark and its effect on her designing capabilities.
All of the Teaser Trailers
However, the one thing that you may have all noticed is that the narrator for each of the teaser trailers states something ominous about the future of the storyline of Season 5.
Twilight Sparkle: "But nothing can prepare her for what comes next."
Fluttershy: "But it'll take more than kindness to face what lies ahead."
Pinkie Pie: "But will the challenges ahead break this pony's sprits? Probably not."
Applejack: "But even though she doesn't know what's coming, don't think this pony's at the end of her rope."
Rainbow Dash: "And while the size of her pride is umatched, her loyalty has no equal."
Rarity: "But, as elegant as she is, this pony knows when to get her hooves dirty."
As you can see, each teaser ends with a line that basically says "The future will hold great challenges that will test the Mane 6." So, what do I think? I think that the Season 5 premiere is going to feature a new villain or problem that will be more difficult to overcome than any other villain in any of the past past seasons.
Can't wait for Season 5!