Perhaps Gary could take Brocks place. (For those of you whom don't know who Gary is, he is Ash's old rival) The relationship between Ash and Gary has grown stronger in my opinion. Gary could want to travel to Issu so he can learn more about the different pokemon there and further his career as a professor. He would end up on tagging along with Ash later on. I would imagine Gary getting some of his old, cocky attitude back. (He will tease Ash and call him "Ashy-Boy", but not as extreme of an extent as before.)
I am not hating on Brock, but i'm tired of the whole "grabs a girls hand and says one corny line before being poison jabbed". If he does stay, I hope he is more creative with how he hits on girls.