I could really go for an increased party size, just because I like their characters and want to be able to play with more of them and hate having to exclude people.

5 would be nice but then I guess that would make it awkward for people duoing the game since one person gets 2 characters and the other 3. Then 6 characters might be a bit much considering the space for combat. Then of course you'd have to rework all the battles to balance for increased characters.