Somebody started a thread on Hydreigon that got me thinking: many of the changes have been bad for it but there is a silver lining.
Obviously, the Draco Meteor/Dragon Pulse nerf, the Fairy x4 weakness and the fact that they completely wall its STABs is not good news.
But OTOH, most Steels no longer resist Dark and a number of them (Metagross, Bronzong, Jirachi, Aegislash) will take SE damage.
Also Hydreigon's movepool is almost endless so it does get Flash Cannon via TM 91 as a way to hit a predicted Fairy switch-in.
So maybe it'll adapt and find a niche depending on how the metagame shapes up.
Not sure how likely this is to happen, but it is certainly a possibility. With steels no longer resisting Dark, Hydreigon could easily get away with running Roost / Dark Pulse / Flash Cannon / Draco Meteor. The only noteworthy pokemon capable of resisting the three moves are Azumarill, Mawile, and Lucario, as far as I can think of. Maybe the sun hasn't yet set on Hydreigon's time in OU.