Since Pokemon: Let's Go, Eevee 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 Misty as the Cerulean City Gym Leader.
Even though it has yet to be revealed what Pokemon (and at which Levels / Combat Power) Misty will own in Pokemon: Let's Go, Eevee outside of her trusty Starmie, many expect her to also have Staryu when in battle. 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. Luckily, since Misty is just the second Gym Leader players will face, her Pokemon shouldn't be all that difficult to defeat with a few Electric Types or Grass Types.