Cinemablend's Pete Haas has reported that Assassin's Creed 3, set during the American Revolution, will deal with slavery. Matt Turner, the game's lead writer spoke about the subject saying:
It's something we've been very aware of. We feel that kind of a subject deserves a certain amount attention because it’s so serious and it needs to be treated with utmost respect. We’re definitely not going to shy away from it in terms of not showing it."
Video games are often not taken seriously because of their difficulty with dealing with "serious" topics seriously. There are a few examples of particularly sensitive or accurate depictions, but more often than not, they tend to become cartoonish in their representation of their themes.
In the Assassin's Creed games, reproducing the cities and buildings as accurately as possible has always been a major concern. They've delivered on that aspect of the games so well that people have been able to recognize places from visiting or living in those cities. But recreating buildings and capturing a sensitive cultural period are different things. But if they pull it off, and depending on how they end up treating the subject, it could mean that more people take games seriously.