On Co-op:
I think people have different hopes and expectations.
I get the impression most people are using "co-op" to mean a multiplayer campaign taking place on the world map, with different factions controlled by different human players likely fighting eachother when there's not a truce. We should be calling it a "multiplayer campaign" instead of co-op, surely?
Pure co-op would be comparatively easy to implement. My preferred way would be this...
One "leader" player controls the party on the world map. The leader and any "follower" players control their own characters during battles, tournaments, at camp and in towns etc. Follower players can potentially amuse themselves with delegated party management responsibilities while the leader is controlling the party's travel on the world map - sort of like a team on the bridge of a spaceship (corny comparision, I know... "Blacksmith, set weapons to full smith... ness. Surgeon, restore that beheaded man to full health."). Each player still has full control of their own character's skills and equipment - they could even have control of their own "division" within the party e.g. their own loyal troops, companions, prisoners and maybe even their own share of the gold. Leadership could be held as a ballot between all the players, or challenged via a duel, or based on the nobility of player's characters, or just agreed on like civilized people who are co-operatively playing as friends can't we all just get along join-hands start a love train etc.
Alright, it's not full-blown campgain multiplayer, but it seems achievable to a noob like me, and could incorporate more continuity, ownership and features than what would most likely be TaleWorld's solution: Joining a battle in another player's game, as an ally or enemy.