Sorry for the late reply.
im thinking of going with mega metagross cos why not? solid switchin to lati too with pursuit and stuff. mega diancie, gardevoir and altaria all seem cool too imo.
also i dont really get the 'efficient chomp/lati' slots thingy, so i should change either one so that there wont be compounding weaknesses? if so, would starmie be a good choice over latias cos rapid spins and muk, and could go with an offensive or defensive set too.
The note about efficient slots is just that in ORAS, defensive synergy is very important because of the variety of threats you have to prepare for (not just in number but in how differently they function and how specific their checks can be), and while you'll inevitably be weak to some things, you want to minimize this as much as possible. Running three Dragon-types is inefficient because they are compounding weakness (and you don't have a standard HO backbone like a Scarfer or PranksterWave Pokemon to really help with that), so making those slots efficient means making sure there isn't an overlap in defensive synergy. Cube, Lati, and Chomp each check some specific things, but on the whole one of those slots would be more efficient with something else.
Mega Metagross is a nice choice over Mega Slowbro, though you'll want to have something for Excadrill. Looking back on it now, I think Amoonguss is more the odd man out than Slowbro (though I think Slowbro is replaceable): it's a complete momentum killer, and you can fit a Dragon/Fairy check into one slot. You might as well be running something like Bronzong, though I think you'd be better served running something more aggressive.
Just to give specific advice instead of a jumble of "you could..."s, I would look to replace Amoonguss and Garchomp. In replacing them, you lose a Fairy check (though Victini helps with this somewhat), your Bisharp check, and Stealth Rock. This means that in these two slots, you'll want Stealth Rock, a Bisharp check, a Latis counter/hard check, and preferably some support for Victini against Fairies. Also, having something that is actually pretty fast would probably help. So why not try:
Clefable @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 160 Def / 96 SpD
Calm Nature
IVs: 0 Atk
- Moonblast
- Stealth Rock
- Flamethrower
- Soft-Boiled
This gives you a nice Thundurus check on top of proving you Stealth Rock and a decently reliable stop to
[email protected] (It can get in trouble if weakened, but you can play around them some with Slowbro's Regenerator.) It also gives you some extra coverage against Mega Altaria, though you're still a bit weak to Fairies overall. It also gives you some status absorption.
The problem with this last slot is that Bisharp and Mega Diancie have very different checks lol. Gliscor might be a cool Pokemon to let you check Bisharp and choiced Volt Switches, but it might be too lacking in immediate power. I will try to come up with a more concrete suggestion, which could change some if you end up replacing Slowbro. (Then I would definitely think about Gliscor.) I would definitely prioritize beating Bisharp, but Slowbro alone is an iffy check to Diancie, to say the least. (This is where your lack of Speed hurts you - you can't RK even the non-RP ones, which limits your in-game flexibility.) Scarf Terrakion is an okay fit.
One poke that might be helpful if ScarfTar, especially if you aren't digging Clefable. It can Pursuit the Latis, RK Bisharp, provide the fakest Zard check ever, and provide a liiittle bit of Speed.