I was thinking that the Elite 4 series could be set in the island where the battle zone and Stark mountain is. Maybe the elite 4 have summer homes or battle facilities (like in the battle frontier)there and go stay there so Ash can defeat them while exploring the island. And the island is probably gonna be larger, like a mini region.
The Battle Zone could be replaced with Poketopia (from PBR) but with the frontier brains from Platinum and the Poketopia Master which could be a smaller tournament arc in which you have to dress more fashinably and be more rated on your strategies than just fight and win, because the trainers use more competitive strategies. It could be, like, Ash arrives in the island, sees Poketopia, but has to go on his E4 journey first. Fight Aaron in the island between Battle zone and resort area (the island will be bug filled), Bertha in the desert,Flint in the survival area or Stark mountain,interrupted by the Stark Mountain sub arc in the middle (Maybe Ash could catch his first legend, Heatran! Or maybe team up with it during the E4 ttournament and then release it, and even with Heatran's help, he would still lose against Cynthia), then continue with Flint, then Lucian in some place between Survival Area and Batlle Zone Then Ash would win the 4 and could take a few days to practice and rest for the Cynthia battle and he decides to go to Poketopia, where the battle style is completely different from Cynthia's (which will cost him the battle), and all of Ash's companions and rivals will participate.
Then Cynthia will be watching and they will go to the top of Mt. Coronet in the Arceus shrine, where the best trainers have battled, as witnesed by Arceus in the sky(The Poketopia "Master's. mountain looks mediocre compared to this, which has a view of all of Sinnoh.) Ash will lose easily, thought the battle will be somewhat long, because it is six pokemon and Cynthia only started using her third one. But Ash will still be happy cause he battled the best of the best, all his friends will be there and most Pokemon fans will not think of him as a noob anymore. To be honest I think they should give contest a break during the E4 ark.
Originally i thought the Elite 4 challenge was like the league tournament and not a new series like most people think it will be but the latter makes sense too. LOL that was a lot of writing, originally going to be a short respond.