Magic Bounce? REALLY? I mean, sure, it's a great ability and all, but we're talking about a Pokémon with a BST of 480 and no held item slot. Xatu almost has the same BST, but has a held item slot, not to mention Espeon and Mega Absol. Good thing Prankster already made Sableye quite usable... man, they really screwed up on this one. It's almost certain that Primal Kyogre and Groudon will get all-new abilities at this point, so while we're introducing some, they really should've gone with a Speed Boost variant that boosts defenses, or something like that. Not Magic Bounce, as good as it is!
Edit: A few more points to consider: Sableye can't switch into non-attacking moves the first time around, whereas Espeon and Xatu get this ability out of the box. This easily offsets any kind of mind game advantage you might have, especially since Prankster Sableye has priority Taunt, which shuts down the same things Magic Bounce does. In addition, I can't imagine Mega Sableye getting powerful enough offensively to go with anything other than Foul Play or Knock Off, especially since its LOWER attack stat is getting the most significant boost. And sure, it seems like it's going to have considerable defense, but rather low HP will force you to max it out to get any kind of reasonable bulk. Add to that the single solitary resistance to Poison to back up those immunities to Normal, Psychic and Fighting, and you get a Pokémon that's relatively easy to hit with strong neutral attacks, not to mention that weakness to Fairy. Also, did I mention no Leftovers? I think I did. Mega Sableye gets no Leftovers.
However, I've seen someone suggest that since Mega Sableye and Mega Diancie both had a gem-based flavor, they might both get the same ability. Today was a crushing disappointment, but if you're not salivating at the possibility of Magic Bounce Mega Diancie, what's wrong with you?