There's definitely more to the story, so I'm sure there will be more games beyond U:</>GA. However, like yourself, I'm not sure if it will be a sequel or another prequel. There is an insane amount of room for prequels - we've seen him meet Elena and Sully, but we haven't seen him meet Chloe or Charlie, and there is clearly at least a little more history in both cases that we haven't had properly explained. I think we'll hopefully see games that cover off such things in the future... I mean, not because they have to, but because there is so much money to be made and there is definitely room for such things without destroying the script of the games set in the "future".