I've seen a few of these, so I'm going to code this while I'm at it. It'll take a while to update since I'm doing homework and such, but here are a few ideas of mine.

Pokemon Villain Teams 4 votes
With a total of Six teams up to date (Rocket, Magma, Aqua, Galactic, Plasma, and Flare), there are many different ways to RP these guys. What would happen if grunts from each team were stranded somewhere and they had to put their differences behind them to survive? There will be a limit on the character numbers (I don't want all of one team and another be empty), but the characters will relate to how they act in-game.

Horses 5 votes
Haven't 100% decided on a plot for this, open to ideas. Don't see a lot of horse RPs.

Wolf Pack 8 votes
Pretty much self-explanatory. I see a lot of these types, so why not put it on the list?
EDIT: No wolf-speak, only real ranks
EDIT: Real ranks

Okami 1 vote
I love this game, though it doesn't get a lot of attention. Why not make a chat where you can be OCs from various places? Events will happen to bring them together.

Mogeko/Okegom
Another thing that doesn't get attention, the details will be brought out later. Canon characters will primarily be used thanks to the heck-ton of characters this person makes.

Any more ideas? I'll add some!!! Votes will be tallied and I'll check back later!