I wasn't around for the first round of Hydreigon discussion-- but this is a really interesting one. On one hand, it's raw survivability stats are... strange. 98 Speed is trolled slow (though 100 base Speed is less relevant than ever before), while 92 / 90 / 90 are decent but not amazing. I guess 92 / 90 / 90 would be acceptable base stats if you a really amazing defensive type like Zapdos, where your unique set of resistances make you a rare full-stop to some really unique threats. In my opinion, Hydreigon has too many common weaknesses-- Dragon, Ice, Fairy, Bug, and Fighting are all pretty big liabilities that add up to being hit SE (or at least for neutral STAB) by almost every major attacker in OU.
On the other hand, it has some really nice resistances-- best of which, the coveted Ghost and Dark ones. The latter of which, adds up to probably the most incredible thing about Hydreigon: It'd be the ONLY Defogger that beats a Bisharp switch-in. Survive the Sucker Punch and SMASH it with Focus Blast / Superpower. Then there's it's great anti-hazard typing/ability to consider, and it's impressive offensive power and healing.
But there's my concern-- it's of course got great BST, but does it have enough to be EV'd to hit hard enough while also being bulky? I feel 92 / 90 / 90 won't take hits well enough to cover those weaknesses without near full investment, which would leave its offensive ability fairly unimpressive.
Overall, I'm not really sold on it (as in would be dying to try this) but just having a defogger in the meta that says FU to Bisharp would be amazing. There's a lot that makes this one interesting, so it'll probably get my vote. IMO, even with Defog Hydreigon's going to struggle to find a team slot, but there's enough interest here to make it a really good theorymon (not Tough Claws Drapion good, but at least more-interesting-than-dry-skin-escavalier good).