While I doubt we'd play as him (at least in single player) I'm guessing the game will explore Liquid's relationship with Big Boss, which is pretty interesting to me.
Yeah, that would be cool to see. Big Boss interacting with a young Liquid also further ties the Big Boss games to the Solid Snake games. And even though he's young, I hope we get to see Liquid fighting. Doesn't look like TPP is going to shy away from showing child soldiers anyway.