I still want to see Chris Pine as a young Han Solo in the rumoured spin-off movie, just to watch heads explode.
As to the "big three", I think only Ford could demand insane money and I think he'd be more interested in approving the script than a ridiculous payoff (just an impression formed from the few interviews I've seen of him with Charlie Rose--not much to go on, I know, but then again I'm only a very casual Wars fan).
I guess this could happen. Fisher says she will be back as Leia, Hamil has been spoken to about Skywalker, and Ford may return as Solo. In any case, the article below says Lucas will have any of the three original characters written out if the actor doesn't want to do it rather than recast the character.
For what it's worth, Fisher is 56, Hamil is 61, and Ford is 70.
Link to story: