Ritchie lost to a nameless player that became Champ (I think)
Ash lost to the guy with Blaziken who then lost to an unknown add in
Tyson won Hoenn's though I don't remember him being introduced before the Tournament.
and in Sinnoh, the uber hacker Tobias won with Darkrai.
Most of the winners were only intro'd for the tournament so to keep the anime going, they'll probably do the same thing. Let Ash get close and then rip the victory away from him.
I also wouldn't be surprised if maybe they do have someone with a tie in like Harrison with Blaziken. So that gives Ash a reason to travel to a new region. Tyson was introduced at Registration and his Meowth prevented the flame from being stolen. It was Morrison introduced pre - tournament.
Hopefully Ash wins, but the way the presented him thus far I don't find that likely. They showed his previous pokemon in the intro, but I don't get the feeling they will be used. If anything they will be used in whatever the filler arc post league is going to be.