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.
Q is more popular and recognizable than the Traveler, and therefore is more of a potential ratings grab.
Finally, why was there never any follow-up on him and Wesley Crusher? Is this something covered in Trek literature?
Wesley Crusher was featured in the A Time To
novel miniseries from 2004 and will be returning in the TNG Cold Equations novel The Body Electric
due out at the end of the month.