2 would be possible if you really planned it out (eg, a frontline fighter with a crossbow and some diverse non combat skills, and a multidisciplinary wizard with healing capabilities), but 3-5 would be ideal, allowing everyone to be specialized without leaving gaps.
This is the idea I had playing DS3 with my roomies. We all love RPGs (we're in a regular d&d group that I GM) and ds3 was the best co op rpg we've found yet, but it still left a lot to be desired. Said two friends are quite skilled coders, and I'm a half decent animator and have managed a handful of creative projects in the past.
Baldurs Gate and Champions RTA were also lots of fun, but didn't even try to scratch the surface of what a coop oriented RPG could be.
That would be the risk. my friends and I play everything together on console because they're macfags
If you can manage to do that, you should be a fucking space shuttle pilot lad
What I thought a neat idea would be, is to have during character creation, each character pick a background trait, such as fisherman, trapper, etc,and how they know the other party members. From there, they would have pre-established dialogues/connections to in world NPCs that would have different dialogues/quest opportunities for them.
The in town/dialogue parts, at that point, would probably all have to be textual to and to count for the number of possibilities.
Rather than grinding, xp is rewarded for playing "in character," eg., a priest praying every night, two characters who are siblings talking about their family while travelling, overcoming challenges in ways that range from combat to clever disarmament to straight up avoidance.