So, here's some of the ideas, gathered:
*The Curator is the Moment, in disguise.
*The Curator really IS a Future Doctor, now a master of choosing a previous face of his.
*The Curator is the above, but at the same time, the same character as Tom Baker in JNT's version of Shada
*The Curator is a splinter of the Fourth Doctor, who went on to live the rest of his life on Earth AS the Curator, before "regenerating" into the Watcher and reuniting with the real Fourth Doctor in the past.
*The Curator is the actual Fourth Doctor from an alternate universe.
*The Curator is the Nth Doctor - the Final Doctor.