Fisher may be far from picture perfect at the moment, but she could easily get back into decent shape to appear in the film long enough to satisfy the needs of her character (passing the reins). If need be, I'm more than happy to see them apply some make-up and or digital effects to help her out.

As for Ford, I think he would certainly agree to appear for a few scenes here and there throughout the series. I certainly wouldn't expect him to return to the series as a main character however.

Hamill on the other hand, with a bit of exercise fits the bell just fine. And I think he would be more than enthusiastic about reprising the role.