>>36738264I guess theres a few ways.
1. boosting their stats a bit, they're all fairly slow, even jolteon feels slow when every mega out speeds him and if you put a choice scarf you do no damage, if he could just out speed mega beedrill then he could volt switch out.
Flareon is the biggest mess, he obviously needs more coverage, a buff to attack/speed, extreme speed would at least help him reach something close to arcanine/entei, until he gets that he will never be close to their level.
Leafeon is similar to flareon, though he does perform nicely on sunny day teams, the speed and swords dance sets him for a good sweep, though he only has leaf blade so pretty much just coverage.
I have no clue how to fix glaceon, seems more dependent on hail teams and ice types being bad.
Umbreon/vaporeon/sylveon/espeon are actually close to OU, they just get out statted or don't have the secondary typing that other choices have to give them good switch ins.
Eeveelutions aren't bad, barring flareon/glaceon, other pokemon just have a little more than what they offer, so unless they get something unique they won't be used besides niche, like with baton pass which got hard nerfed.