Hmm..the American patriots don't seem to be the enemy in ACIII so it doesn't make much sense for the Spanish Empire to be the enemy in this one since they were allied to the Americans during the revolution..
However if that's the direction they're taking this game in then I assume the main Templar enemy would have to be Alejandro Oreilly because he was the Spanish governor of Louisiana that crushed the rebellion this game takes place in.
But on a deeper level who is this main character defending? The French? I doubt she'd rather be under French rule since they had Louisiana in worse shape. If she's fighting for freedom then why would she work with the rebellion that aimed to restore French Imperial rule to the colony...not freedom...either way I'm interested in finding out when it's released! :)