It sounds good but Patty idea sounds like Ashley from RE4Thanks. Yeah, Monster Mash-Up is probably one of my favourite too. Glad you like the idea. You pretty much helped me come up with it, so thank you =)
Playable characters in this game so far are as follows (In terms of normal story missions).
Patty Lowell (Yes, hear me out) I was talking with Richtofen, and we came up with the idea of making Patty a playable character since she's a main character in the Anime. She won't be able to fight demons, obviously, but her missions will be more based on avoiding danger, solving puzzles and following/tailing people. This introduces a different variety of levels, making it not just purely hack and slash based (But primarily it will be).
Mission wise, I'm not sure on how many there will be at this moment in time. But more information will be given when I come up with more ideas.