So I just joined up yesterday, and as a long time visitor to the site, I figure I should get started and begin participating. Here are a few ideas I've had in my head:
1) Dorothy from the Wizard of Oz vs Alice from Alice in Wonderland(both would be adults of course). Perhaps, Wonderland and Oz begin crossing over, and the girls blame each other for it, or something along those lines. Lots of clothes ripping, rolling around and all that other cat fight(and sexfight) stuff ensues.
2) Princess Peach vs Princess Zelda. If you're a long time video game fan(and you've played the Super Smash Brothers games) then you might have considered this one at some point. I'm thinking a weird idea where Peach and Zelda come to blows over who gets kidnapped more often. It actually leads to them trying(and succeeding) to kidnap each other, with the one in control constantly switching as each one tries to be the kidnapper instead of the kidnapee. Lots of clothes ripping, rolling around and all that other cat fight(and sexfight) stuff ensues.

3)... oh. There is no three. These are free ideas, so anyone who feels creative can give it a try.
And now, I'm out.