I just got an idea I fancy very much.
What if BBC secretly shot some open-ended footage with Tom Baker intended to be randomly discovered around the 100th anniversary that could be fit in any multi-doctor story?
that the Curator could never appear again, because if he is a future or the last Doctor, it makes no sense to feature him in any story (well, it might
make some sense for Trenzalore, but other than that...) Unfortunately by the time any story involving him makes sense, he'll sadly no longer be among us. So it basically can't happen. We won't ever know his character is up to.
Unless BBC prepare with some footage beforehand.
(Obviously, won't happen, but the thought gives me a pleasant sensation)