What is it with the assumption that the Pokemon Trainer is suddenly going to change his Pokemon selection, or even be replaced himself? Acknowledging the Pokemon Trainer is indeed Red, the only logical course to venture upon is to have the Trainer use Pokemon he already possesses, and of course, acknowledging Red, there would be no reason replace such a man of elevated status. Consider these points of contenion:
1. Replace the Trainer - Red
This variation of the Pokemon Trainer (Red) is considered to be the most iconic, especially in terms of being recognizable with not only the Pokemon but the Trainer itself. Why replace a quintessential aspect of the Pokemon universe with some unknown, never before heard of, God-knows-where-that-came-from Trainer from Kalos? Not unless Red is playable in Pokemon X and Y, yet to my knowledge he is not.
2. Replace the Pokemon - Squirtle, Ivysaur, and Charizard
Consider the complicated process of Sakurai having to balance out not one new Pokemon, but three new Pokemon. Why not, instead of rebuilding the structural foundations from scratch, simply just take what is already before us and modify it to an extent that it works within its new surroundings. With the Pokemon Trainer using original three main Starters, Sakurai can already implement an underlying blueprint he has used previously in balancing the roster, but with a few major changes here and there; of course, there's removing the stamina system (which I doubt, though), buffing certain aspects of each Pokemon, and balancing out with newcomers. In substance, having the current selection of Pokemon would work just as comfortably and conveniently.
I will consider the idea that two Pokemon Trainers can co-exist... yet, that all depends on whether or not Sakurai will devote such energy to a project such as that. Personally, I would favor the effort towards three other characters, whether veterans or newcomers, than to another Trainer. That is just my subjective opinion.
It's not to say I wouldn't enjoy a new Trainer or even new Pokemon, but to transition from the legendary status of Red and his Pokemon to something else entirely less (at least in exemplary terms)... that I cannot envision.