He has already achieved what he has set out to do when he first started racing, to win a F1 drivers championship.
But of course F1 also has its negatives, as Hamilton has experienced in his short career already. Too much politics and no matter how talented you might be, you are not getting anywhere unless your team manages to build you a good car. And with the ever changing regulations, you don't really know what you will end up with. There is of course all the hate thrown towards him. I've seen him use the autosport main page. I wonder if he ever visits the forums. If he has then he obviously is made of sturdier stuff than me because if such vitriol was thrown against me I would have called it quits ages ago.
So where can Hamilton go? A lot of places. Personally I would love if he went to NASCAR. It would be much a lot of exposure for both NASCAR as well as international racing fans. On top of that his background (race) would bring a lot of attention as well, in the same way Tiger Woods did for golf.
There is also the option of going back to GP2 and doing what he did best. I don't think he would enjoy Rallying because it's not actually racing, but more of a time trial.
Hamilton to NASCAR please.