I feel like Mega Gyarados has the potential to be a top tier Overlord in OL&UL.
Its pre mega Water / Flying typing gives it access to a plethora of strong followers, with lots of dominant flying types and UU being littered with waters.
Plus mega Gyarados itself is a very strong set up sweeper that could potentially crush teams single handedly in a predominantly UU environment.
Here is a list of potentially strong followers:
Water: Suicune, Feraligatr, Empoleon, Crawdaunt, Swampert, Alomomola, Tentacruel, Slowking, Jellicent, Cloyster, Milotic, Qwilfish, Seismitoad, Sharpedo, Gastrodon, Kingdra, Omastar, Quagsire, Blastoise, Poliwrath, Vaporeon.
Flying:
Salamence, Zapdos, Crobat, Tornadus, Gligar, Mandibuzz, Aerodactyl, Yanmega, Fletchinder, Honchkrow, Moltres, Noivern, Togetic, Xatu.
LOTS of options offensively and defensively. Plus the typings work together quite well as the Flying types beat the grass types that trouble the Waters, and the Waters can take on Ice types. The Electric weakness shouldn't be too bad thanks to access to all the Water/Grounds and Gligar (which I'm assuming is a good electric check cause I'd guess most electrics run HP Grass > HP Ice given all the water/grounds in the tier). Besides there aren't that many good electric types in UU anyway.
Mega Gyarados teams just seem ridiculously versatile, with sooo many strong pokemon that can be thrown together for anything from full stall to HO. I find the concept of Mega Gyarados stall in particular rather intriguing as I believe it was actually a thing in OU for a very short period of time when Stall teams started using bulky Mega Gyarados to counter Baton Pass teams. Plus in Ubers Mega Gyarados has a RestTalk set that seems to beat a crap tonne of the UU meta, though there is plenty of cleric support available so maybe Rest Talk isn't even necessary, which opens up room for moves like Toxic, Thunder Wave, Scald, Taunt, or just more coverage to hit a certain mon you wall.
But of course offensive and balance teams would also be extremely powerful, with Megadose itself being a crazy sweeper, and some high order teammates like Salamence and Feraligatr leaving little hope for the opposition in defending against you.
Also one thing that I noticed which could potentially be stupid but is interesting none the less; Swampert, Sharpedo, Blastoise and Aerodactyl are all considered viable in UU without their mega stones, but their megas are all still legal and of a higher rank in UU too. Plus regular Gyarados is A+ in UU and isn't really set back should it lack an item, so I'm wondering if perhaps another one of these could be megas could fair well enough without an item to consider a double mega comp. Say Aerodactyl + Gyarados for instance, then depending on whatever would serve you best in the given match up, you could either mega Gyarados for an OU mon and a B- rank UU mon, or mega Aero for an S rank mon and an A+ mon. Its probably stupid but I couldn't help but think about it because I don't play UU and am not sure what items these pre megas run or how reliant they are on it.
Like I said I don't play UU so don't take my word on everything but I still expect Mega Gyarados to be a really good leader in OU&UL and wanted to contribute as its a pretty sick meta.