Firstly, who do you think would win in a battle, the Traveler or Q? Both are very powerful, and can manipulate time and space, and the traveler is more benevolent.
Secondly, why was he never brought back for Voyager? He would have been a common sense guest star for that show, since he can travel long distances easily. They took advantage of this for the use of Q, but not for the traveler.
Finally, why was there never any follow-up on him and Wesley Crusher? Is this something covered in Trek literature?