The point I was trying to make is more that if you're trying to add a Pokemon to your team that has Magic Bounce, and you don't want to use Espeon, you have no other choice but to use your Mega on a Magic Bounce user. The use of the Mega for that purpose doesn't fit everyone's needs, and that's why Espeon is still used in a lot of cases instead of Mega Sableye and Mega Diancie.
...Except it's
not used in "a lot of cases", and there's data to back that up, too.
Also consider that there's a Mega Evolution with Magic Bounce for basically every playstyle (stall, offense, balance, etc.), so you can typically fit one of them into any team without too much duress (not every team though, but that's being overly specific). Unless you're building a team around a specific other Mega Evolution, and somehow "need" Magic Bounce, which honestly, you don't, because as nice as Magic Bounce can be, unless you're utilizing Baton Pass you typically don't build a team around Magic Bounce. Baton Pass teams are/were literally the only things where Magic Bounce was specifically required, and in that case, since Espeon didn't have the Mega Evolve turn where it was without Magic Bounce (meaning Boosts could be Baton Passed directly into Magic Bounce), it was the Magic Bounce user of choice.
Magic Bounce is a perk, not a necessity. There's very little in the way of teams that benefit from being built around Magic Bounce other than the aforementioned Baton Pass teams, which got neutered into oblivion on Smogon. And by that logic, without its Baton Pass teams, Espeon's barely worth using.
I would just fill Espeon with a bunch of special attacks like psychic, shadow ball, dazzling gleam, signal beam which is good coverage against its weaknesses. It's as frail as Jolteon but less speed so I would give it a focus sash
Focus Sash Espeon is literally just an inferior Alakazam, who runs the same set with more power and speed, but thanks to Magic Guard has an easier time keeping its Sash intact.
There's also Dual Screens since it can't be taunted (barring Mold Breaker), and can set them up against Stall with ease thanks to Magic Bounce.