We must remember that Button is a championship winning driver, so he doesn’t necessarily need a ‘superstar’ teammate.
Schumacher might be an option (imagine that..!). As would Rosberg (although i doubt it, as I think he has a long future at Mercedes). Kubica would be a gamble – and many think that Ferrari is his ultimate destination.
More realistically though, I could see Glock, Hulkenberg or Di Resta moving there. As mentioned, Button is a proven race winner. He may not be massively rated by everyone here, but he’s certainly good enough to lead the team on his own while a younger driver is trained up to be a title contender in the future.