This is my first post here, and I'm also not sure if this is the right place to post this (so please tell me about it if I'm wrong).
Anyway I'm developing a 2d side-scroller for the iPhone. It's a game with high scores (but has a new concept too), and I was thinking of implementing unlockable characters in my game. So instead of coming up with useless characters, I was thinking you could offer your characters as unlockable ones (with credit, of course).
For more info, contact me at samghobril(at)gmail(dot)com.
Firstly, welcome to GDNet, and this forum is fine for this particular request.
I think the best way to approach this particular idea would probably be to find some specific games with characters that interest you -- probably smaller games rather than smash-hits, but it certainly won't hurt to try with some of those as well -- and contact the creators directly with your request. If you're polite, your game sounds fun, and you ask enough people you're likely to find at least some who are willing to offer up the use of a character or two; if you've already started working on the game, showing a fun video or a couple of screen shots might help to show that you're serious.
You might of course also find some people who just offer up characters in topics like this one, but I think the above suggestion might increase your chances.
Hope that's helpful, and good luck with your game!