On Celestia in this picture, I can think of a few things you could've done:
1. Solar Flare, since she's basically to Celestia what Nightmare Moon is to Luna.
2. Trollestia/Molestia. I'm not sure if I would like this as much, but they are technically evil versions of Celestia. Plus, it'd be better than leaving Celestia unchanged for this pic, like somepony else suggested (Me: ).
3. Discord. I know he's a guy, but he's kinda the most fitting canonical villainous foil to Celestia, like what Chrysalis is to Cadance and NMM is to Luna. Cause, thinking about it, he's a malicious prankster (contrast to how Celestia uses mild pranks to deflate the pompous and break the ice) and he also messes with ponies' minds and spreads disharmony by giving them advice tinged with a lethal dose of cynicism. Not to mention, that plan of his to render the Elements useless by breaking up the Mane 6 with his "advice" is tantamount to Celestia's not-so-evil genius. Just evil, obviously.
That aside, I love this! These mares are far too cute, I can't resist them (maybe that's their evil plan?)!