I have no idea who would be king. I think the Sparrow's plan is for Tommen to be stripped of his kingship since he's not really Roberts child and with the church and kingdom being combined it would give power to him. He's playing the long con here. Acts like he doesn't want anything except to worship the gods, but he's actually playing everyone and is about to be at the top of the damn food chain. None of it will matter though because Cersei is going to burn him alive before she dies. The Mountain may do it. He's going to turn someone into a puddle of goo, it's just a matter of time