Full Idea
If I say 'It will not be me, but one of my future selves', I do not imply that I will be that future self. He is one of my later selves, and I am one of his earlier selves. There is no underlying person we both are.
Gist of Idea
One of my future selves will not necessarily be me
Source
Derek Parfit (Personal Identity [1971], §5)
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.219
A Reaction
The problem here seems to be explaining why I should care about my later self, if it isn't me. If the answer is only that it will be psychologically very similar to me, then I would care more about my current identical twin than about my future self.