Many players like to do missions, others dont, or dont want to repeat them with a 2nd, 3th chr. I think it would be good to give alternatives to bypass missions that are necessary to get to the next part of the map by giving adventuring alternatives. What I have in mind is this:
Allow players who kill bosses to find keyparts to the next area. When they found all keyparts they unlock access to that aerea just like the mission would have done.