This is a multi-faceted question,
teabagyokel with some assumptions thrown in that (a) Lewis will leave McLaren, (b) for Mercedes and (c) for the 2013 season. I think (a) is almost certain but I will leave my doubts aside about (b) and (c) and how it hinges on Vettel to Ferrari (in 2014
or 2015). Perez's future is also determined by Vettel to Ferrari, I think
This is really important for me because I think Sergio has a good claim to the McLaren drive, and would be McLaren's choice without Perez's 'ties' to Ferrari - whatever those ties mean
.
If Vettel and Alonso are at Ferrari in 2014, Perez needs to be in a McLaren or a Red Bull. If that is really going to happen then going there for 2013
if when Lewis leaves for Mercedes, is a really good option. Or he could wait for Seb's seat in Adrian Newey's car. If Vettel isn't going, then Sergio will be at Ferrari in 2014 and maybe even 2013. His future is bright - as long as he keeps having races like Malaysia / Monza occasionally.
Leaving that aside, I think Perez would pair well with Button and allow McLaren to design a car that suited both drivers and allow them to go a particular direction with tyres for example that didn't compromise one or other driver. Commercially, I think Sergio is probably also McLaren's best bet with the Americas a decent place to start looking for new sponsors.
The other driver who would make a good team with Button would be Rosberg but I cannot see him running away from a confrontation at Mercedes with Lewis and Mercedes have already made it very clear he will drive for them next year. I don't think Rosberg is available for 2013. Neither are Vettel, Alonso or Webber of course
Kimi is a possibility, especially based on talent alone, but with Kimi and Jenson, I feel that the team might struggle to find something both drivers like. Because of that, and because we all know that Kimi would hate the McLaren corporate BS all over again, I don't think that will happen.
Then there is the Force India pair. For me now they are neck and neck. Hulk has his DRS open but Paul has the inside line approaching the hairpin at the end of the season. Paul is very good when he has the inside line into the corner at squeezing his opponent out of road, but Nico is flying at the moment. It could be tight and they might even collide with so much at stake. They may also have both heard over the radio that Perez is leading them both because he has a better tyre strategy.
The wildcards are Kobi and Grosjean. Of these two, Romain is the more likely multiple race-winner but both could be in contention at the end of the year
if when the decision to replace Lewis for 2013 has to be made. I very much doubt McLaren will move quickly, nor do they have to commit for a long term - they have a very good hand in any negotiations. If Lewis does leave then maybe they will also start asking Vettel about his contract with Ferrari and whether he might instead prefer Woking for 2014.
Who will be at McLaren next year?
Lewis for one more year Paul di Resta as everyone waits to see what Vettel does.