Sailing, damage, boarding in that order.
A finely simulated and integrated sailing/damage model would be the next gen feature that has never been achieved before, apart from in the crudest terms. Weirdly enough, also having an arcade like mode for easy playability for non grognards would hugely increase sales.
Boarding is essential, as it is such a huge part of the reality that this game attempts to simulate. The level of verissimilitude is the big question, mainly governed by technical/financial constraints on development. I'd vote for a high level, as this could be very salesy.
A full complement of ship types is obviously a DLC opportunity. I'd love to see the key types at launch, particularly 74s and frigates.
Ship's boats, stern boarding, false colours, letters of marque, bring em on when you can!