I'm pretty sure fanfiction.net has a few of those already. I'm writing a sorta/kinda crossover myself! I call it "
Pokemon XD: Impostor of Darkness"
But I suppose certain characters would be all over certain pokemon (Alice would love Shuppet and Banette, as would Medicine. I could see Marisa taking up with a ton of different pokemon, Reimu would get a Meowth [Pay Day], Sanae would be all over Elgyium, Flandre would probably try to get all the Fire Types, Remilia would get the ones that love the night, The Prismrivers would go after Ghost Types, Kanako would probably love to get Groudon, Suwako would try to get Kyogre...) The list goes on, but odds are, the characters would all have favorites or matches, and go after them. (I could see Mima going after Shedinja.)
What if I'm too lazy to ask a question and want someone else to ask one?