Please bear in mind I'm still trying to test this (let alone get the team balance right in general, since OU is not the main tier I play), but I finally got around to testing my manual weather hydreigon I came up with
Hydreigon @ draco plate
Levitate
Modest 252 SpA, 252 Spe, 4 HP
-Draco Meteor
-Fire Blast
-Sunny Day
-U-turn
The concept of this was that hydreigon matches up pretty well against all opposing weather starters, in the sense that they will switch out rather than risk losing the weather war, so I decided this would be a good way to turn the weather match-up for sun, since only scarf variants of either tyranitar or politoed will dare risk confronting hydreigon (even then, that's a maybe). Outside of that, the few walls that will dare to take on hydreigon I will U-turn out of, either to dugtrio in order to trap them, or to sawsbuck in order to set up on them/eliminate them with coverage.
Dragon/Fire is pretty good coverage in OU anyway, with the only resistor of both types in heatran would be very hesistant to switch in for risking being trapped by dugtrio. Draco plate is to power up the Meteor since fire blast should be boosted by sun, while not wearing out hydreigon too fast from LO recoil.
Other notes
-Using a move over U-turn such as EQ or Tailwind may help more, as sun can be a little slow, or if you want to kill opposing ninetales straight up rather than have them roar out a sweeper later on (may require some EV changes to do that).
-In terms of team-mates, dugtrio is good simply due to trapping heatran (though it might help me more if I used venusaur over sawsbuck, since sawsbuck is already accomplishing roles hydreigon is trying to do). A Set-up sweeper that fares well against walls such as the blobs, jellicent etc would also be a good way to take advantage of hydreigon as well.
Like I said, this is still being tested (asides from questionable team-mate choices, I'm also facing a large amount of weatherless/opposing sun on the low end of OU's ladder rather than rain/sand), but I guess this is a potential support set, while still being able to send mons running in fear.