Since Pokemon: Let's Go, Pikachu is, in more ways than one, a revamped remake of Pokemon Yellow, it was expected that a number of core aspects of the original game would continue to be in place. One things most fans expected to remain the same was the original Kanto Gym Leaders, and this appears to be so with the confirmed return of Blaine as the Cinnabar Island Gym Leader.
Even though it has yet to be revealed what Pokemon (and at which Levels / Combat Power) Blaine will own in Pokemon: Let's Go, Pikachu, many expect him to have the likes of Rapidash and Arcanine in battle with him. These are the same Pokemon he had in the original First Generation Games (as well as the later remakes of such), and one should expect and prepare to face these same Pokemon. Blaine could also own a Growlithe and Ponyta or a Ninetales, depending on whether his team will replicate that of Pokemon Red & Blue or Pokemon Yellow.