Is there a way to allow players to use their 3 main mp characters and another npc charcter? So they can do their 3 and then 1 additional 4th npc that's on the list?
Not currently. I think it would be cool if your active chars were added to the monster dropdown list.
This is something that the custom scenario builder really needs. It would open up so many possibilities and enable us to make all kids of cool and creative scenarios. Now when you want to be creative, you need to use various tricks and workarounds, and many things are just not possibile.
It'd be great also if there were actual NPC non-enemies—so, for instance, you could have doors that NEITHER player can push with Team Shift!