I'm sure everyone since the start of the game has wondered if there would be another class and it is possible for another one, but what would your idea be of a new class? Not counting corsairs and marauders, a made up name and what they do.
Let's look at this from a "Holy Trinity" of roles standpoint. For Damage Dealers, we have Musketeers, Swashbucklers, and Witchdoctors. For Tanks, we have Buccaneers. For Healers, we have Privateers. Because of this, I would tend to say that a new tanking or healing class would be optimal. Preferably a healer, since we can never have too many of them. If it were up to me, I would go with a staffy weapon wielding, heal using, cleric-style class.