For me it really depends on the game. Some games I feel don't really need any added content, for example Mass Effect but if there needs to be one I think I'd prefer a prequel. In Uncharted there's a great deal of potential for either a sequel or a prequel as one can imagine Sully and Nate having further adventures and we know that there's a lot of their history that we don't know about.
Seriously, let the franchise be for a while, think of a new trilogy....a trilogy well planned out and then do a new one when the time is right.
Prequels should be left for handhelds in my opinion.