It's sad but I think eevee will never get another evolution...

Why?

Well... I've evidence to back this theory.

1. Extreme Evoboost.

The description of this move in sun and moon is as follows:

After obtaining Z-Power, the user, Eevee, gets energy from its evolved friends and boosts its stats sharply.

So... if eevee gets more evolutions, extreme evoboost's animation would be outdated as the new eeveelution wouldn't be in the animation.

This seems to show that the Pokémon company is kind of done with eeveelutions.

This is further backed up in

2. Sword and Shield

Pokémon Sword and Shield, also known as generation 8 games, didn't have a new eeveelution.

Yea, we got gigantamaxed eevee, but it just isn't a eeveelution. In fact, it's an illusion, According to the principles of dynamaxing and gigantamaxing from what I remember.

So? It's only one generation fluff, it's not a big deal.

Well, I've more evidence

New eeveelutions have a trend of being added in even numbered generations... besides the original trio of course.

To date, not a single even number generation skipped the introduction of an eeveelution... Besides generation 8 of course.

Gen 2: espeon and umbreon

Gen 4: Glaceon and Leafeon

Gen 6: Sylveon

And yea, even the number of eeveelutions being introduced has decreased.

Oh well, right now I can only hope that this theory is wrong...

I kinda like eeveelutions, even though eevee themselves are the best.