Hydreigon's main role is this of a wallbreaker, and it struggles to do this in OU. Checking Aegislash is a nice perk of Hydreigon, but the nerf to Draco Meteor's BP and Fairy-types mean that Draco Meteor isn't the ''destroy all'' button it was in 5th gen and Hydreigon actually needs to predict between Fairy and Steel-types. The buff to Dark Pulse is nice though, giving to Hydreigon a decently strong spammable move that 2HKOes most of OU when followed by the correct move (Draco Meteor, Flash Cannon, or Fire Blast). Also, Hydreigon now has 4MSS, as it has to choose two of Fire Blast, Flash Cannon, and Superpower. Could end up either bottom OU or UU.
As for Haxorus and Salamence, they are both lost causes in OU. Salamence used to have three good sets, mixed, DD, and Scarf. The DD set is completely outclassed by Mega Char X while also having big 4MSS, the mixed set cannot wallbreak anymore and thus is not viable anymore, meaning that the only one still worth using is the Choice Scarf set on DragMag teams, which are not so good atm anyway. Haxorus can still wallbreak fine thanks to Poison Jab, but there are just better wallbreakers out there, such as mixed Aegislash, Mega Lucario, and Mega Garchomp, and with the end of 5th gen, noobs will stop using Haxorus just because it looks cool. Both Pokemon will fall to UU almost surely imo.
Oh, and regarding Starapatror, i think that it finally has a chance to not be broken in UU. Doublade, Mega Ampharos, Metagross, Tyrantrum, and possibly Klefki (which i think will drop to UU once people stop using it just to dick around with Prankster) are all solid checks or counters to it that can fit on all kinds of teams, and the introduction of powerful priority users that can OHKO or almost OHKO Staraptor after SR and/or a bit of recoil, such as Aqua Jet Crawdaunt and Mach Punch Breloom, will also definitely help keep Staraptor in check. So, between those Pokemon and Zapdos, Bronzong, and Rhyperior, it's very possible that Staraptor will finally find a place he can call home, if of course my assumptions about those Pokemon ending up in UU are correct.