Hopefully Hamilton does leave McLaren. McLaren needs a driver who act responsibly, think independently, take responsibility for his actions and is a team player. The team's wage bill would be a bit cheaper as well.
Paffet or Davidson 
I'm not sure Paffet is really good enough to step up to a racing seat. De la Rosa would be a better short-term option. Davidson has shown he's maybe worth trying out. Di Resta is in the McLaren driver programme, so I'm sure he'd be thinking he deserves a chance. Whilst it's tough to say how well he would do in Formula One, he's shown enough promise to warrant a crack at the whip. I would like to see McLaren try to lure Vettel, Alonso (albeit unlikely in those cases) and Rosberg to the team.
Having watched Paffet in DTM, I think he'd do well in Formula One. His race-craft is certainly good enough. Alonso in a McLaren again would be nice to see, because he is talented enough to bring McLaren several more titles, but could you imagine the people in charge (and more importantly, Mercedes) going for that?
In fairness, if Lewis left mid-season, the seat should rightfully be de la Rosa's for his loyalty and because he's a solid driver. But in the longer-term, I'm not sure whether or not that would work out. Could Pedro mount a successful challenge?
I think Vettel will be contracted to BMW Sauber, still, and so it might be hard for McLaren to get him. And with Williams' pace this year I would imagine that he will want to stay with them (and I'd like that - if they build a car capable of fighting for the championship then he would certainly deliver). I do, however, think that Mercedes would push to have a German driver in at least one of the seats. But I wouldn't really consider Sutil for the seat. Rosberg, Vettel, Glock or Heidfeld would do a good job in a McLaren. Perhaps a German driver and a British driver would make a good combination. Heidfeld and Di Resta would make a good pairing with experience/consistency from Heidfeld and raw speed and aggression from Di Resta. Or Vettel and Davidson, with the roles the other way round.
And then there are the non-Germans to look at. Alonso has already been mentioned, Webber wouldn't be bad, Kubica, even Massa. There are also some decent drivers in test seats at the moment worth considering.