I'd really love a multi co-op option in this game !!
Co-op games are far too few those days and it really adds a lot to a game to be able to play it with friends (online or not, LAN is even better !!)
I played NWN in co-op and it was really awesome, i really hope something will come out in this one.
Solo shouldn't be forgotten of course, co-op game must be a kind of add-on, you choose to play that way or not.
NWN multi system should be chose as base system and maybe making something new and exciting from it.
Co-op system should be just some programming to add to the solo mode (maybe some stats adjustment, drop random system and some other things ...) nothing out of budget but i'm no developer so i can't judge work it needs but it really adds to the gameplay and i would be the first to play it co-op only
No need of some maps/quests/campagne editor that would be out of price, just some co-op mode to start with and editor if funds are enough.