Nnnnnnnoooooot really tbh, I'm less familiar with Mummy but Promethean is by and large about an intensely personal journey of self-discovery and self-actualization. A small group of people, constantly moving through a world that hates them, trying to build themselves into the people they want to be. It loses something pretty important when it's generalized into Being A Part of the Kitchen Sink. Like even if you're going the Dresden Files route and just throwing a ton of stuff into one city (and I love Dresden Files for all it's problems tbh, it's what got me into UF in the first place) even then Prometheans should be these weird fringe things that are more passing through and not well understood than a part of the familiar terrain. And Mummy, afaik, still has pretty much all the same problems it has with non-Mummy stuff if you mix them together.
I could see it working if you cross them over in a "ships in the night" sort of way? In a Promethean chronicle, a Mummy plays a central role in a single story arc as this mysterious, eldritch thing which shows there's strangeness in the world beyond that birthed by Divine Fire; the way Mummies tend to pop up out of nowhere, slowly burn out, and then disappear again, carrying all this bizarre history and knowledge with them, seems excellent as a hook - and there's also the element of them being former humans who became this deathless thing by choice, opening a door for certain PCs to hit a milestone or two.
From the Mummy side of things, the Created are this alien OCP they have to try and process through the lens of the insane Lovecraft shit that Irem was involved with - are the Prometheans the result of someone fucking with stars (again)? Are any of the creepy Great Old Ones they have to deal with (either directly or, more likely, via one of the Deceived) taking an interest in these almost-humans? Is there any sort of connection to be drawn between Irem and the cycle of demiurge-to-Promethean-to-Promethean creation - or is there a Lineage whose genesis involved a Relic, either one of the OG five or an entirely new one introduced for the sake of the chronicle?
What would one of the Mesen-Nebu Guildsmen think of how the precepts of alchemy were applied (or misapplied) to capture sparks of Divine Fire and cage them within dead flesh?
Hhhhhhheeeeennnh is it though? Hunter already provides, like, antagonist versions of player gamelines to work with, enemy Hunter organizations, and just general weird shit. And pretty much every other splat has their own tailor made antagonists, usually with a fairly wide spread of options. Changeling has Huntsmen, Loyalist Lost, True Fae and their Titles, plus hobgoblins and mortal agents. Geist has necromancers, Cthonic Reapers, hostile ghosts, enemy Bound, etc. Mage has the Seers, the Abyss, Left-Handed Legacies, Banishers, That One Fucker Who Keeps Trying To Blackball You In The Consilium. Vampire has, most principally, Other Vampires. I mean it's all...if the best thing you can say about Beast's utility is that it provides niche antagonists that work worse than something out of Summoners or Intruders or Predators (any one of which can also provide Rad Shit that's by and large gameline agnostic) that's not exactly a ringing endorsement.
Beast is useful as a source of creative grist for the mill.
I could totally see a Hunter campaign involving people who fed their souls to eldritch spirits of fear and madness, gaining terrible power in exchange for having to sate the spirit's hunger with a steady stream of victims.
I could see a fucked up cult whose senior members can invoke the Labyrinth their dark god hides at the center of, swallowing up enemies into a shifting maze which actively seeks their end while the occultist fueling it hides away in its depths. The only way out is to either kill the person who called it forth, or use sacred geometries and other paranormal means to find a path out through the Labyrinth's ever-changing passages (but if you do, what else might escape that dark place by following in your wake?)
I could see a coven of witches who learn of the Sons of Cain, those twisted kin of humanity who inspired legends like Goliath and Grendel - dead to the last centuries ago, but still wandering the world as ghosts, for neither Heaven nor Hell will accept them. They seek to bind one of these restless shades to a mortal vessel, gaining a minion who partakes of these lost giants' unbelievable strength and vitality.
Hell, you could even go for a take on
The Keep, with the Beast being a sort of semi-MacGuffin sealed away in some lonely corner of the world, uncovered by the Loyalists of Thule or somesuch and now oh shit, those guys we sent to investigate that ruin all went crazy and killed each other, better send in a fresh batch to investigate that! Cue everyone getting spookifered in an antediluvian monster-jail whose occupant is trying to convince them to let it out.