The OP's list is pretty good already (especially with the additional suggestions in this thread), but there is yet another thing one could add to the list:

Being able to see your coop partner's actionpoints/how much are left after each action would be nice. This could be simply indicated by a number under the portrait (not on it like it is with the left rounds for summoned beings) or small dots below it, maybe even a segmented bar - or a number/bar below the circle during a combat or alike. Not badly necessary, but a nice feature or option that one could enable at least. When playing solo, I know how much I have left for each of my own characters, but this would make the actions of the coop partner a bit more translucent such as making interaction and cooperation better e.g. if I know he has 3 points left and I just remember I could send him my potion to use in this round so he can survive longer. This would be very useful during hard fights at least.