Yeah, the main co-op obstacle is the turning system. But a system turned based can be modified in a Real Time (turned based).
Is not so easy to explain, i’ll try.
Actual system:
Turn based on player will.
All events are independent, but forced by the player will on turns.
New System:
Turn decided by an amount of time (secs)
All events are independent, follow turns will.
Player can do or not do something during the turn time.
In detail:
One can decide that a turn last 1 sec. If you do something in this 1 sec, you’ve done something in that turn. If not, nevermind. The turn ends and all events do what they have to do instantly. Next turn.
The main problem is that the game, now, end a turn only when a player gives an imput. (walk, sleep, craft, attack, etc).
If turns are seconds and runs in backgrounds, each action may needs a certain amount of seconds and turns will pass in real time.
Is not an easy mod, i see it, the main problem is the turnation system, but i think that the actual code have all the data to add this.
Fixed this, the cooperative mod whould be just The Game as it is now, in a lan (or hamachi) private server, like starbound and others.