Jolteon used to be pretty good, Flareon have a nice look but its awful. Umbreon and even Espeon are being reduced to relics of the past.

I really want to hear from you how improve these mons.

Imo, all Eeveelutions need three Abilities, and these Abilities must be really good to make Flareon, Jolteon, Leafeon, Umbreon and Glaceon more competitive.
In some cases, only a new Ability does not help, Umbreon and Flareon for exemple need a better move pool.