Some cool things with ORAS:
Mega Metagross:
Monster in the wings. Still getting meteor mash, as well as knock off for coverage, this is an utterly fearsome dragon dancer. With blaze kick in the 4th slot, it hits almost everything for SE damage and hits extremely hard with tough claws boosted moves. No Psychic Stab, but it doesn't want it with its 4mss anyways. Fearsome sweeper imo.
Mega Lopunny: Close Combat + Last Resort best set imo Close combat is much more reliable than high jump miss, and although it doesn't gain setup, its great speed tier and good coverage are to be feared as usual.
Mega Slowbro: This thing just got really annoying. Gen 1s movepool gives it leech seed just to be that much more annoying, this alongside spore/twave/toxic, confuse ray, and maybe metronome if thats your thing. Twave+confuse ray + Leech Seed + Recovery can be super annoying to face because of the huge bulk.
If you really love water/ground typing, Mega Swampert is your go-to. A bit weird, but can run DD or rain dance to good effect (and if finally gets recovery in slack off for your defensive mon)
Mega Diancie love spamming stuff like diamond storm, precipice blades, and play rough; its physical set is now very good. Lay down some webs and this thing can be a nice pivot (it gets parting shot/ks as well)
The rest of the megas either got shafted or are not that useful, so leaving them out. If Mamp has stuff I missed look out for his post.
Precipice Blades, Dragon Ascent, and Origin Pulse are 3 new really good moves for gen 6 mons, although I have too much to do to list out some sets for them.