Notice how Lee Kelso (in "Where No Man Has Gone Before") looks like Connor Trinneer. So perhaps Kelso is a descendant of Trip?
IIRC, Kelso was supposed to have grown up in an orphanage. So he could be a Charles Tucker VII or whatever, and not even know it!
And I am firmly convinced that Jack, the kid from ENT's "Carbon Creek", is actually Jackson Roykirk, who will one day build and launch the Nomad probe. We never learn Jack's last name, do we? And Jack is obviously a smart kid and very interested in astronomy...
Also I have a theory that ENT's Admiral Forrest is actually Flint, from TOS' "Requiem for Methuselah". Not because they look alike - although they kind of do - but it's just that Vaughn Armstrong is my favorite actor of all time, and I was rather upset when Forrest was killed off. So any plot twist that allows him to live, is fine by me!