Er...Tsukihime, A Drug That Makes You Dream, Kanon, Tales of Symphonia, and Devil May Cry 3?
Well, I imagine Shiki, Dante, and Mai would probably bond over the whole demon-slaying bit...at least, once Dante realizes that Arcueid, Ciel, and Akiha are off-limits for flirting (not because of Shiki, but because they'd pulverize him, themselves). I imagine if Kohaku could spot her drug habit and get her some training (probably from Regal and Dante), Nekoko could join in on the action with that cattle prod she carries...assuming the combined humanity of Colette, Kouhei, Raine, and Lloyd isn't enough to consider that plan cruel and inhumane.
I don't imagine Aeka, Mizuki, Hisui, Makoto, or Shiori would have much to do. Even Nayuki has the stamina to keep up with some of the other, more superhuman characters.
...actually, you know what? If we can give all the Kanon heroines their Eternal Fighter Zero abilities, something tells me they'd manage.
Also, Yuuichi, Mizuki, and Kouhei would probably become a comedy trio to take Tethe'alla by storm. Aeka and Hisui can still keep each other company, and if Kouhei and Shiki aren't enough to keep any prospective monster from messing with them (assuming Hisui doesn't have her Melty Blood abilities), they still have Genis, Sheena, and Presea as bodyguard detail.
Zelos would probably be in a coma during all of this due to female overload.
...
Needless to say, this would probably be an action RPG.