Evil!Celestia: Like others before me, I don't see it. Or rather, I can surely see the technical possibility, but for the kind of story the show is, it would actually be a bad move. Not all stories need cynicism on that level. Hence, the creators of the show won't do it and shouldn't do it.
If a story contains a seemingly benevolent authority today, LIKE FUCKING CLOCKWORK the fandom will start create theories, no matter how epileptic, to explain who they are REALLY evil or tyrrannical or liars. This is easy, since true benevolence is kinda hard in the real world, there are always side effects, always other views, and because of that it is hard to do in stories. I think there are a fair amount of Evil or Corrupted or Tyrant Celestia fanfics in ED; seems to me be the most common epic fanfic.
I actually think many girls would buy an evil pony princess doll, but then it should have to be evil-cool, not all pink and shit. Something like Nightmare Moon or Midnight Devilwitch Bling-Bling:
http://yamino.deviantart.com/art/Midnight-DevilWitch-BlingBling-199291069
;) (Girls, like boys, do the weirdest shit with their toys, after all. I know one woman who more or less decided her Barbie-knock-offs lived in the world of Thundarr the Barbarian and had to defend their little suburb against her brothers action figures, and one whose Pocahontas scalped her Barbie...)
So if they do Tyrant!Celestia she should have a transformation sequence into a cooler doll. Hey, then the girls' parents will have to buy TWO dolls!