Obviously, excluding Pikachu! :P
Yeah all of ash's pokemon are seen again at the pokemon league meaning they are high level, it would be a great disadvantage for the gym leaders if ash just came out and used such powerful alReady 3rd evolution pokemon plus the creators of pkmn also want to incorporate the new pokemon from each region in ashes team.
Yeah, actually this happened twice, in the same region, with the same pokemon. The pokemon was snorlax in the johto region. Ash used him in the sumo contest and the blackthorn gym.
No, because the show probably said this: it would be totally unfair if ash would use a really high Level Pokemon in a new region. BUT since pikachu is his first pokemon he should keep it.
Pkmn gamer was semi-correct Ash took his aipom with him to sinnoh from hoenn though he was a stow-away if I recall