Visually I think Black Flag still holds as it is, plus - the story is what keeps you replaying it.
Sure, upgrades of visuals are always pleasant, but in this case I don't think it is necessary. I would rather see development of more DLC:s for it as the setting and foundational story is so compelling that you just want more and more stories and plot.
Not sure how though. I would say maybe exploring the years of Haythams childhood/Edward training him and later years to becoming a templar would be a natural expansion of Black Flag into AC3.
