Personally I'm on the fence about the original cast (possibly) returning.
I'm also surprised there's so much ageism going on here - Alec Guinness was in his early 60s when New Hope came out so I guess that means he was 70+ when ROTJ hit theaters? I can't tell you for certain if he was healthy or out of shape 'cause he appeared in all his scenes wearing that cloak! If Mark does come back, he could easily hide his appearance in a similar matter. The possibility of Luke Skywalker for SW 2015 is not that absurd.
Hamill could play Boss Nass with almost no makeup...
I met her, fifteen years ago; I was told there was nothing left; no reason, no conscience, no understanding; and not even the most rudimentary sense of life or death, of good or evil, right or wrong. I met this cow, with this blank, pale, emotionless face, and the blackest eyes... the devil's eyes. I spent eight years trying to reach her, and then another seven trying to keep her locked up because I realized that what was living behind that cow's eyes was purely and simply... evil.