There is totally a difference between temporarily going mega (something already temporary to begin with) without all the bells and whistles and temporarily evolving though. It's like the suggestions I've seen of Pikachu evolving into Raichu for the final smash. Reverting back would just be weird in a way that mega evolution isn't. One's a more acceptable break from mechanics than the other.
Also Vaporeon's tail is like twice as long as the others, much less Jolteon's tail nubs. The tail would need to be ignored for attacks if we just want it to be a shulk-styled thing, which just seems awkward.
Which is all to say I've thought that a fighter that permanently evolved into a better version of yourself but enough about sheik for the rest of the battle would be kind of interesting, and Eevee in particular having 3 avenues would be the ideal. Like all of them would share basic moveset, but specials would operate differently (flareon gets fire breath that works more like charizard, vaporeon gets a long reaching hydro pump with high end lag, jolteon has an even longer reacher thundershock that comes out fast but deal piddly damage and so on), hitboxes would be of different sizes (ie all have an up titlt with the tail but vaporeon's has the longest reach while jolteon is just a stab upward), stats would be different (flareon is slow but high damage & knockback, jolteon is very fast, vaporeon is a heavy boy) and so forth. But also this involves making 4 semiclones (or at least melee-styled) of each other with the base form just straight up being worse and that seems like a lot of effort for a gimmick fighter.
secondary thought was a fire emblem character whose gimmick is promoting into a better version of yourself.
