When you get right down to it there is literally nothing stopping the writers from giving him a Sinnoh pokemon if it's promoting D&P remakes. Sure some people try to say that oh Ash will only catch pokemon of the current generation to promote current generation and that the writers would only give pokemon that advertise the remakes to rivals or companions, however in all reality it doesn't matter if some people think that it would be better for the companions to be the ones to promote the remakes, as really it's up to the writers to decide who will promote the remakes, so if they decide Ash will get one gen IV pokemon on his team then he will regardless of what anyone thinks.
Also keep in mind that the writers have been pretty unpredictable in Sun and Moon so if we do get D&P remakes then you can't just assume that it's safe to say that Ash won't get a gen IV pokemon, as they might decide to give him one since so many people seem to think they won't give him one, of course this is all if we get D&P remakes this gen, which at this point we don't really know if where getting D&P remakes this gen or not. Also keep in mind that since they have already had cases where a companion or rival had pokemon that advertise the current remakes, it would be pretty predictable if a rival or companion this gen got the pokemon that advertise the D&P remakes.
So when you get down to it considering how unpredictable the writers are being this generation, they could easily give Ash a gen IV pokemon to advertise the D&P remakes which most people here are honestly not expecting.
I agree with you that ash can get a Gen IV pokemon if writers want to. But at this point of time we can make some predictions as to what can happen and what not in the anime.
People say writers are unpredictable in sun and moon. They should remember that writers have always been unpredictable. They have always tried to show something new since AG. This is not something exclusive in sun and moon. Take for instance, writers didn't gave ash all the three starters which was unpredictable back then. In DP, he got two starters, which again was something new. He also got a pseudo- legendary officially. They gave him more than 6 pokemon to ash in BW, new again. A gym battle with uneven number of pokemon on both sides, it used to happen in games but was new for anime. And ash losing to cameron in a 5vs6 battle, that too was unpredictable. Don't forget they skipped grass type in XY. And many other similar instances can be given.
But even if they are unpredictable, there are some things we can always be sure of i.e. we can say that its safe to say that this will happen. Like if ash travelling to a new region, its safe to say that he will have a full team by end. Another instance, if the new region requires a character to win eight badges before entering a league, then ash would win those eight badges and will compete in league i.e. writers won't make ash unable to get 8 gym badges and therefore, cannot compete in the league.
Similarly, i believe it will be safe to say that ash's full team in a new region would consist of only the new pokemon and not the old ones. I think the same is for team rocket. They too should not get the previous generation pokemon. Logic is, they don't give old generation pokemon to characters that have already visited that region earlier. For instance, in AG, it was may who got bulbasaur and squirtle and not brock or ash. Similarly, in sinnoh, dawn got cindaquil, again neither ash nor brock.
If they really want to promote Gen IV pokemon, then there are other ways to do that. Like companions and rivals. If they want to involve ash in the promotion, they can make him use his reserves like gible and other sinnoh pokemon in some kind on battle tournament and other similar arcs. Nothing more than that.
But still, writers are unpredictable and can do anything that can break someone's predictions in seconds...
