I have a feeling Eccleston in like five years time or so will really wish he returned for the 50th. Unless of course it's crap, but I do doubt that. But if the show's still on the air and he's not aged too much, maybe we'll see him in the 60th. But unfortunately, I think he'd have aged too much. Even Tennant might be too old by then.
It makes you wonder what would have happened if someone else didn't turn down the role of the ninth doctor, I think Hugh Laurie and Hugh Grant were offered it. Would they have returned? Would they have even stayed on longer? Or would the same story with Eccleston have happened?