There's the Buffy the Vampire Slayer/Forgotten Realms Crossover, which is well written. Except for Ranma 1/2 outta fucking nowhere that has no bearing on the story or plot whatsoever and can be completely ignored in favor of it just being an Asian foreign exchange student and nothing changes, and the slight Social Justice ques that pop up from time to time.
Still needs to be a d20Wizard in 40k, an Inquisitor in Ravenloft (2e) A Space Marine in Hell, and a Chaos dude in the Abyss. Or some of these dudes in Eberron. And Illithid in 40k would work also. A VTMB Vampire of any sort with randomly generated backstory would be neat if dropped in D&D, as now it's free from Gehhnna for one thing.
Quake is CoC is a must, because you know, literalyl killed a manifestation of dear Shub-niggurath, so pairing you up with Old Man henderson would be the nest best thing.
Solider from TF2 In anything is of course ripe for fun- 40k, any D&D setting, Dragonlance and Birthright would have him fit right in, Merasmus would be busy getting drunk off his tits with Vecna or somesuch, meanwhile Demoman Steals a Gith Ship, takes a silver sword and becomes a spacejamming Astral Pirate with a Legacy Weapon Eyelander.
A Warcraft Lich in D&D would also be interesting, as he runs on Necromancy not tied to the plane of Negative energy, and has many advantages over Atypical Undead, namely the fact that the speed of transformation shit's all over what's available in the planes- dropping him in Icewind Dale might be fun.
Skeletor is literally a Yugoloth, but popping him as a Mercenary in the Blood war would be neat. Also, there's ripe room for a Tomb King in the Forgotten Realms, because of Sandstorm and Dry Liches and the Egyptian Pantheon/ Mulohandi (Literally KANGZ, but I guess one can make do with that as they establish a glorious desert empire)
Legacy of Kain:Ravenloft would be the tits, Nosgoth get's added to the Demiplane of Dread, Local ecology mixes.