Will Smith comfirmed to Collider (here) that a prequel to I Am Legend is being considered:
I Am Legend
We have a fantastic prequel idea…we’re still trying to work through a couple of bumps in the story. It’s essentially the fall of the last city – the last stand of Manhattan. The movie would be… within the body of the movie D.C. and then Manhattan would fall as the last city. It’s a really cool idea trying to figure it out…there’s a reason why we have to take a small band and we have to get into D.C. So we have to make our way from New York to D.C. and then back to New York.
I Am Legend was a great and complete movie by its own. I don’t think that a sequel to I Am Legend would do good… People would go see the movie sequel, no doubt, but they would most likely come disappointed from it because it would never fit with the vision they may have built of what may have happened before I Am Legend…