Rumors are placing him in McLaren-Honda, but only if he could get a 1 year compromise, so he could go to Mercedes when Hamilton's contract is over. Otherwise he'd get a sabbatical, which OTOH doesn't seem a great idea. My guts guess is that Mercedes is going to manage pissing Hamilton and he'd leave right this very season back to McLaren. No rational cues to think that. We'll see, Mercedes just assured the constructors title and probably that one of his pilots wins the drivers', so why not making a German driver to win it.