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.
I really don't get these kinds of arguments. You guys do realize we are living in an electronic age, right? It would be easy as pie technologically to allow, as an OPTION, an increase in party size up to six when in single-player, and to restrict party size to four for co-op play. No "rebalancing" of anything will be needed, because, to repeat again, this would be an OPTION. Any player exercising this OPTION would be accepting that encounter "balance," whatever that means, will be upset by increasing the party size, and it will be up to them to decide, for their game and their enjoyment, whether this is acceptable to them. There is nothing technologically difficult about doing this at all.