I for one think that Billy was an awesome character and would love to see him come back. There's so many possibilities. They could do a side story that tells about Billy's time in Africa. They could do another game that takes place at the same time as RE 2 and/or RE3 telling about his escape from Racoon City. He might even be in a game like RE 5, which I know that that possibility has already been discussed.

As much as I love Billy, I seriously doubt that Capcom will bring him back in a future installment of the series or a side-story. He's definitely NOT going to make an appearance in RE5, and I think that Capcom is pretty much done with his character.