When the first CoroCoro scans leaked a few days ago, some people were able to figure out that Mega Metagross would get Tough Claws, despite how blurry the scans looked. This has been officially confirmed, meaning Metagross basically gets a free Life Orb boost on every move it commonly uses bar Earthquake (at the cost of getting hit by Intimidate if you care about that, I suppose). YES PLEASE. Perhaps more important, though, is the fact that we have details on the stat boosts it will boast, with a significant boost to speed as the highlight. And with 100 points to distribute in five stats, if Speed gets the biggest cut it will definitely be enough to land it in triple digit territory. We've seen the good it did the likes of Gardevoir, Medicham and Lucario, so if anyone still has doubts on Metagross' return to OU, I don't know what can convince you. Also of note is the fact that every stat will get boosted, including some wasted points in special attack. Oh well, this thing looks like enough of a beast already, we don't want it to be sent to ubers before the end of the year, right? Still, remember that Metagross is currently out of the top 100 in OU, and is in the lower half of the UU tier, so it absolutely needed help.
As for Mega Diancie, a drastic shift in stats has been suggested, with "extremely high attack and special attack" and "blindingly fast attacks" offset by drops in both defenses. Considering regular Diancie has 100 in both attack stats and 50 in speed, something tells me we're going to be dealing with very significant boosts, especially in speed, which means the defense losses won't be as marginal as Garchomp's speed drop (from a numerical standpoint, I know that speed drop cripples Mega Garchomp greatly in practice). And since Carbink and Diancie have both been all about multiples of 50 so far, my educated guess is that Mega Diancie will be having 50 in HP, 100 in both defenses, and 150 in both attacks and speed. Basically Deoxys-RS without the obscene movepool, but with workable defenses and freaking Magic Bounce. Speaking of which, it really does look like Gamefreak really wants to avoid making an incredibly bulky Pokémon with Magic Bounce, hence this major shift. Otherwise we would've seen a Mega Diancie capable of setting up some Calm Minds with impunity, then putting on the hurt with its admittedly few special attacks.
Now, with that stat shift in mind, how would such a Pokémon be used? Calm Mind sweeper anyway? Maybe use that speed to set up Stealth Rock and screens like Deoxys-E did? I'll be honest, it's hard to say, for the simple reason that ORAS will likely bring about several tutors, and some of them are bound to help Diancie. So it's definitely not guaranteed that Mega Diancie will be going in with nothing but Moonblast, Psychic and Ancient Power on the special side, and Diamond Storm and Return on the physical side. Besides, it's a Pokémon with a BST of 700 and Magic Bounce of all abilities. People will find a way to make it work somehow, that's a guarantee.