The more time it takes, the more content and new stuff will be in the next branch.
They should seperate people/teams working on bugfixes only and 1.x.x version only and people working on new content and new stuff like 1.x.0.
Honestly implementing COOP into singleplayer game is stupid idea. I think games should be first implemented with coop in mind. I guess it would be great like in a 5 years(when they first finish a game) as a standalone product focusing purely on coop. The best as i would see it that each player is independent lord that can do whatever they want with map etc - but their game is kept in sync by server, and it should allow to some amount of players - like 8-16, obviously this would mean that battle sizes would need to be reduced drastically - because i doubt that any server would for example handle 4 battles with 1000 troops each or it would require like 2 ryzen epic CPU, so i guess providing such server to be installed for people wherever they want could be good enough, but as i say, first make good single player with similar amount of content like warband at least, decent multiplayer and then focus on coop seperately(using already good content from singleplayer).