@Ziggyfin: There might be a slight possibility that it could happen, anything can be possible it is all dependent on the creators of the anime. One reason that Ash might leave his Pokemon with the new professor is that since the Isshu region is far away it might be harder to transfer his Pokemon and so he decides to take the Pokemon with him to the professor. But also since we have seen Ash in the Best Wishes trailer he is battling with Pikachu and we don't quite see his other Pokemon. Also his Pikachu gets sick whenever they get to a new region and after Pikachu gets better then Ash visits the Pokemon center so all that time Ash has to carry around his Pokemon with him. Overall there might be a slight possibility that Ash might take his Pokemon to the Isshu region for easy access for further use of his previous Pokemon or he might just keep it at Professor Oak's lab since he is already at the Kanto region.
