Feebas

Evolve it and have a nice day.

There are some insightful thoughts that Gen 3 Mythical research may have a quest requiring walk with your Feebas 20 km and evolve it to Milotic, just like the Gyarados quest and Eevee quest in previous 2 Mythical line.

I'd hold onto it for a potential jirachi special research. With all the mythical Pokemon there are niantic will likely need to put them out faster.

At this point, just evolve it an enjoy. It's unlikely that the rebalance is going to significantly reduce how useful Milotic is as a Gym Defender, or increase how much use it is as a Gym Attacker/Raid Attacker (which is pretty much no use at all).

Regarding the idea that it might be useful to keep one around for Mythical Research quests, we thought the same thing with Celebi. That we would have to evolve a Wailmer or a Swablu or a Feebas as part of getting Celebi, and instead we got Evolve an Esperon and Umbreon. So at this point, I don't think it's useful to keep any Pokémon unevolved just for the sake of having them be potentially useful for a Mythical quest.

As for Community Day and Milotic, I don't see that happening. Feebas has a 20km walking requirement to evolve, and there's no reasonable way to work that into a Community Day style event. The most I could see is having an "Event" like the Psychic event where every Feebas evolved got a special move, and Feebas were spawning at the same rate now as we see Drowzee or Abra, but even then I don't see that being a workable event for Niantic.

Feebas is a pretty unlikely CD mon because it has a special evolution requirement and only evolves once. It also doesn't really have much potential to be much better than it is currently, Hydro Cannon is limited to starters and outside of that, Milotic already learns the best water moveset.