Supernatural/The Walking Dead, I'm guessing. TWD is a very very good zombie apocalypse TV series (based on comics/graphic novel of the same name), though I'm not sure how things are gonna work now that showrunner Frank Darabont (of the Shawshank Redemption fame) is gone.
Anyway, I've read several promising HP/SPN crossovers and I can imagine how SPN/TWD would work well together. So, an HP/SPN/TWD fic? Definitely doable. Zombies are not that far off from supernatural elements and the magical undead, right?
Right now I'm imagining a kind of super!zombie apocalypse sic'ed on Britain that's immune to pure magic and normal ammo (short of nuclear blasts that evaporate everything, these zombies are here to STAY). The TWD folks supply zombie-fighting strategies. The HP peeps sups up the Muggle fighters and supply medical backup. The SPN characters give an extra umph to normal ammo with their supernatural talents, thereby making them being able to affect the super!zombies somewhat. And Castiel's an Angel. That's gotta count for something.
How did the super!zombies appear anyway, and why are all three 'verses meshed together? Well, the world government released a series of nuclear bombs to destroy the TWD zombies/the Muggle Britain gov received wind of Voldemort's activities and decided to nuke the entire Wizarding community to get rid of him and blame the blast on the Russians (is it the Russians in the late 90s?)/the SPN boys accidentally a nuclear bomb (c'mon, it's Sam and Dean and Castiel *rolls eyes*). These simultaneous blasts thinned the three worlds' space-time fabric so much that the worlds meshed together, and the zombies were boosted up to the next power level during transportation. Or maybe they were affected by the magical energies of Wizarding Britain. Or something.
Unfortunately I've never watched SPN and I'm not familiar enough with TWD to write this. Anyone want to take a stab?
(Grr, dying comp. I think I just about killed it again by typing this out ;___;)