>>54198945The real answer is that there's probably some shitty ass coding in the Pokemon games to where they'll shit themselves if they have to change the total number of evolutions a mon has from 8 to 9. There was some extreme jank in certain romhacks to where they effectively had to create a Eevee.2 in order to include new evolutions because the game simply wouldn't function otherwise.
On a more theoretical level, what type is there left to explore?
>Dragon/Bug/Ghost/Flying/Steel/Fighting/PoisonThese types often are baked into the basic design of a Pokemon if they're a mono-type, and would be a little too "out there" for an Eeveelution (if you look at the basic designs of these monotype pokemon, none of them would really fit what Eevee is), which is probably why we haven't seen them and won't ever see them.
That leaves Rock and Ground. I don't think we'll see a Rock-type Eevee with Lycanroc being fairly popular, so the only real hope out there is Ground, which has a history of cutemons (Sandshrew, Cubone, Phanpy).
