Regarding the Ash Greninja, the first thing that came to my mind when I saw it was spiky-eared Pichu. Obviously we're dealing with much more drastic design shifts here than just one different ear, and it may well be a Mega, but I feel like -- particularly with a Pokemon as popular as Greninja -- they'd go a little more all-out with a Mega Greninja.
That's just my initial reaction, though.
As for Megas I want...I'm honestly not really dying for any, most of the Pokemon that I'd be interested in seeing a Mega on are Pokemon I'd rather get a regular evolution. But if that's out of the question (and I have concerns that it is) I want:
- Mega Tropius
- Mega Castform
- Mega Maractus
Since my knowledge and interest in the competitive side of Pokemon is extremely lacking it's more for "I like this Pokemon and think they have some untapped potential" than anything. The former two are two of my favorites, Maractus is just a Pokemon that hasn't really caught my attention but it's a cool Pokemon in one of my favorite gens so I'd like to have a reason to pay more attention to it.