If they throw enough money his way, I'm sure Mark Hamill will be up for playing Luke for a new trilogy (keep an eye on his Twitter account!).
Otherwise, recasting wouldn't be out of the question, especially if there is a large gap in story time between the OT and ST (as there was between the PT and OT).
In Disney I trust. They are doing a good job with the Avengers movies and I am confident they will do a good job here. At least the franchise is now out of the hands of George Lucas.