If it's truly like Hoenn all over again, the rival(s) will have Delphox or/and Greninja
However I should point out even though the rival(s) did have Pokemon matching either Ash's Pokemon or the starter weak against Ash's starter because Blaziken was not seen because May likely had a Combusken, I'm not sure if it will actually happen exactly the same.
Since the type the starter is, is switched with Ash's male traveling companion, it's likely because Clemont does have Chespin that it will match up in the league with Chesnaught and Greninja.
I think that's a closer match up, than because Ash had the starter stronger than one of his rivals.
So it would make sense to be Chesnaught and Greninja, and it would match up in Hoenn.
It just depend on if you believe that it would match up because Ash had the stronger starter, OR if it was because both male main characters had the starter Pokemon.
I want Ash with Greninja, anyway, so screw some rival having, I'll be pissed when that happens.