Now, before I continue, just remember. This is my personal opinion. If you disagree, that's fine. Just show some respect for it, and I'll respect yours.
If you ask me, I think a crossover game/anime/manga/comic book between Rosario + Vampire and Street Fighter would be awesome. I could picture these rivalries easily:
Ryu Vs. Moka
Ken Vs. Tsukune
Dan Vs. Kurumu (Allure Vs. Super Taunt)
Q Vs. Mizore
Sagat Vs. Kokoa
Inner Moka Vs. Evil Ryu
What about you guys? What rivalries would you think would be good?