EqD posted this S4 teaser, spoiler warning, I guess.
I'm not really happy about the "Rockodile" villain or whatever the hell it is, but I am interested about the part at the end "There can be only one princess in Equestria!" Despite my urge to make a lame Highlander joke, this plot point really intrigues me. It was Luna's voice, wasn't it? The idea that the princesses are being corrupted is a really cool idea, and I'd like to see where they go with it. I watched barely any of Season 3, but I think I'll be watching the Season 4 pilot, if only for that part.