Not to turn this thread into a single argument but want to counter some of your points. I also want to preface that I'm not an expert player but I have played ADV for a long time, I also wouldn't even be opposed to a Lati@s test but I don't think it would be unbanned.
I should also mention I'm mainly going to be assuming Latias in my arguments, because I think Latios is even more bonkers and doesn't warrant discussion. I also want to note Latias is incredibly versatile and can often choose what it wants to beat with it's wide movepool, including some niche but potentially useful moves like reflect/charm and surf to beat tar, etc. It can go max defenses and refresh to beat wall counters like blissey too. However I think most will run enough speed to beat base 100s, enough spat to hit benchmarks with it's attacks, and the rest thrown into spdef. I'm also going to assume it's running lefties, sandstorm is up, and it has at least some moderate spikes support. Assuming the main sets are:
Dragon Claw | Tbolt | Calm Mind | Recover
and
Psychic / Dragon Claw | Recover | Roar | Random coverage or status move here
what about blissey or most calm mind users?they can easily take those attacks.
Blissey is certainly an option, although it's worth noting that the latis can easily 1v1 blissey if they opt for a max hp spread and refresh, but like I said, I don't know how common that would be. You're right, Blissey does beat most of the sets. Blissey beats a lot of things. Blissey is very passive and gives free switch ins to a huge portion of the tier. Good teams will always have ways to capitalize on Blissey, including just throwing dugtrio at it. Blissey also is not the type of mon that can be thrown on any team. Also, calm mind users won't have a calm mind up when they switch in? Do you mean they can switch in and win a cm war? Because the only OU CM users are raikou, suicune, and celebi, and I don't think any of those are beating Latias in a CM war.
it doesn't sound much scarier than a calm mind suicune.if anything it is less scary since calm mind does not give it physical protection.
Suicune might have great physical bulk, but it doesn't have recovery outside of resttalk, which is really bad this gen. It's vulnerable to all of TSS. And it doesn't have an EQ immunity or fighting resist. Latias can switch into CB EQs and BBs and turn that into momentum at no cost. It isn't chunked by swampert EQ switching in.
that's not necerily a game breaking thing,there are pokemon that can run speed boosting moves and outrun it and pokemon that can slow it down with paralysis.
Being outrun by pokemon that have used speed boosting moves is not a negative, because those pokemon have to switch in and use their speed boosting move, and while they do that, Latias can switch to a counter? Or are you saying it doesn't beat a pokemon that boosted on something else, and uh, yeah, it isn't meant to be a wall that stops all sweeps? Also, the only speed boosting moves in ADV are DDance and Agility. Agility is seen on Zapdos, who isn't beating Latias, and metagross (if agiligross still exists) who does admittedly beat Latias. DDance is seen on Mence and Gyara, who are never setting up on Latias and can't even threaten it at +1 ( +1 252+ Atk Salamence Hidden Power Flying vs. 4 HP / 0 Def Latias: 211-249 (69.8 - 82.4%) -- guaranteed 2HKO after Leftovers recovery ), and tar. So you do give setup to Tar and Meta, but a lot of pokemon do, and good counters exist to those pokemon. Both also need to scout before trying to set up. This isn't even mentioning that if the Latias user can get 3 layers of spikes up, it only takes on roar predict to neuter both these mons.
vaporeon can theoretically work too
I did the math and I found out that a vaporeon with 16HP,240 sp.atk and 252 sp.def with calm nature which holds nevermeltice can 2hitko both of them and survive 2 thunderbolts(my calculations were based on the fact that in ubers they only boost special attack and speed EVs,also I did not add sandstorm in my calculations.)
Damn, you just have to run the most ridiculous Vaporeon set ever to beat Latias, all good. First of all, you're running a no physical bulk vappy with no leftovers. This vaporeon is going to get dugged, or at the very least, you've made your team really weak to DDtar. Nevermeltice is a joke of an item, giving up lefties in ADV just for a 10% boost to your coverage move? Also, you need to have haze/roar here, or else even a bulkless latias can just set up in your face - 240 SpA Never-Melt Ice Vaporeon Ice Beam vs. +2 4 HP / 0 SpD Latias: 81-96 (26.8 - 31.7%) -- guaranteed 4HKO after sandstorm damage and Leftovers recovery. Thats what happens when Latias clicks CM as vappy comes in, then again as it ice beams. Without haze/roar, you're left fishing for crit/freeze. BTW just from 1 layer of spikes and two turns of sandstorm (one switching in, one while you ice beam on their 2nd CM) your vappy has lost 24% and dies to +2 tbolt. So you put on haze/roar - except now you're running surf, ice beam, and roar, which means you aren't running wishtect, and you now have a vaporeon with no recovery lmao.
Let's do a comparison. Latias shares a lot in common with Zapdos. Well rounded fast special attackers, with a spammable stab move, good defensive typing giving them a ground immune but not a sand/toxic immune, and their counters are all grounded and vulnerable to roar with spikes up. Zapdos has for a long time been considered a top 5 pokemon in ADV, and I've even seen people say it's top 3.
Now take Zapdos, add surf and ice beam, refresh, a very relevant 10 base speed, no rock weak, good support moves like screens and wish, calm mind, and RECOVER.
Again, I don't think Latias would be unbeatable, but I think it would be incredibly centralizing and can sometimes just win because it decided to run max bulk refresh into your stall team. But more than anything else, if you want a reason not to test/release latias: there isn't a need to. The ADV metagame is fantastic as is, and I see no reason to try and throw a wrench into it.
I am pretty sure that the metagame would adapt in order to deal with them.
If your idea of adapting is ever team running blissey or an atrocious vaporeon set, I'd hate to see what you think is too centralizing.