If it was a new Eevolution though, there are only so many it can become:
Normal(the thought of it becoming stronger but maintaining its Type has been floated)
Poison(Unlikely to happen but typically when an Eevolution is introduced, it's done in pairs and with one having an advantage over the other one. Exception being the original three Kanto Eevolutions. So you had Dark/Psychic in Johto(Dark > Psychic) and Grass/Ice in Sinnoh(Ice > Grass), but no counterweight to Fairy Sylveon when it was introduced).
Steel(Ditto Poison but unlikely to happen)
Dragon(This honestly has more legs than people realize. It could evolve via a Dragon Fang, hence would be an item based Evolution. Furthermore, Sylveon would dominate this new Dragon Type).
Flying(It's a possibility but one that doesn't seem likely)
Ground(If Steel is unlikely to be done, then this won't be)
Rock(Ditto Ground)
Ghost(not sure how this would work...)
Bug(Ditto Ghost)
Fighting(Yeah... not going to happen)
So knowing this, if they were to add a new Eevolution, I'd be leaning Dragon Type. However, based on Ash's reputation when it comes to Evolution, it's got a higher chance at staying as Eevee. It also falls in line with Let's GO premise of not evolving your starter. Since Pikachu won't be Evolving, this Eevee won't be Evolving either.