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 Sabrina as the Saffron City Gym Leader.
Even though it has yet to be revealed what Pokemon (and at which Levels / Combat Power) Sabrina will own in Pokemon: Let's Go, Pikachu, many expect her to have the likes of Kadabra and Alakazam in battle with her. These are the same Pokemon she 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. Sabrina could also own an Abra if based solely upon Pokemon Yellow, or even Mr. Mime and Venomoth if things are based more upon Pokemon Red & Blue.