While I'm not particularly interested in the co-op mode myself (I prefer strictly single-player in my RPG's), I do have an opinion on what improvements Larian might have planned for the save game control.
In D:OS, it wasn't a big deal for just the host to have access to the save, since it was only two players. However, with D:OS 2 - and now four players as well as GM mode - it would make sense to have an option to setup your client as a dedicated server; so that even if you're not playing, others can still login and play their own characters.
The host would still have control over the save game, but wouldn't have to actually be playing the game for fellow co-op players to have access.