What if Buffy failed to stop Glory, the barriers between dimensions were ripped asunder, and a megacrossover ensued?
Once the fabric of reality was ripped asunder, Zelretch realises what's happening, and uses his magic to assemble a team of archmages from the worlds rapidly blending into his own, and with their combined powers, they stabilize reality before it's all destroyed. Ed Greenwood, Elminster, Mordenkainen, and Raistlin, from the DnD worlds. Lofwyr, Ghostwalker, and Harlequin from Shadowrun. The Doctor. Doctor Strange. Autocthon. Vector Prime.
So, reality is stabilized, but are just getting started. Hordes of Tanarri and Baatezu are loose on Earth, and busily turning the world into the latest battlefield of the Blood War. Meanwhile, TITAN warmachines and Exsurgents are busily killing, uploading, and infecting people (and not always in that order). London is being torn apart by a force of Nazi vampires. Every sort of Gundam is waging their war of space people vs Earth people. Fortunately, heroes have arrived as well. The Exalted. The Justice League. The Justice Society. The Avengers. The Technocracy. Firewall. The Counter Force. The Slayer.
Conflict is hardly limited to Earth, either; the Green Lanterns don't look kindly on interstellar empires, and come down on the Shiar, Kree, Skrull, Ori, and Galactic Empires like a ton of bricks; Tarkin is not amused when the Death Star's superlaser is blocked by a Green Lantern's power ring when he fires it at Alderaan. The Ori are a somewhat harder fight, being godlike energy beings.
Throw in a bit more conflict between the local forces of the United Federation of Planets, the assorted warring Gundam factions, the Earth Alliance, and the assorted Eclipse Phase factions as well for some action in the Earth's local, and radically changed, neighboorhood. Not only has the Earth acquired Eberron's dozen moons and ring system, but Cybertron, Autocthon, and Malfeas hang in the sky like metallic moons.
I'm not sure what to actually do with this setting, but it'd probably make a kickass roleplaying setting.