As a reader and dabbler at writing fan fiction based solely on a specific game or genre I always enjoy the well thought out crossover, for example say Fallout and Stargate SG-1 where one of the Daedalus starships experiences hyperdrive problems and emerge in the Fallout Universe... so what type of crossover do you feel would go with the Fallou Universe

Fallout and Stargate SG-1 -

Fallout and World of Warcraft - somehow through technology or spell an expedition emerges in other universe

Fallout and Battletech/Mechwarrior

Fallout and Robotech/Macross

Fallout and Mass Effect - Humanity rebuilds and extends cautiously to the stars finding the Prothean ruins and cache of tech but does not fall blindly into the same tech path as the Citadel races

Fallout + Bioshock

Fallout + TES same concept as magic

The thing is, it's not the idea it's how you present it. Give it a good backstory, and explain things probably then most will like it. Though that's the hard part.

Very True Vatrou... as to the Fallout WoW crossover I have a brief outline for where the United States in the hope to shortening the time to transport troops and supplies to China another group of scientists tweak the Big Mountain Transportalponder tech resulting in a Matter Transport or Mat-Trans chamber that sends a group, platoon + maybe battalion with and experimental MCV Yard or Mobile Construction Vehicle Yard to Azeroth. Because of time differences the surviving members spend 25 years or one year for every four that pass in the Wasteland in Azeroth...

I think it's be more interesting to just start a Fallout-like RP.

Perhaps the mass extinction event occurs in a world that is much more advanced than ours. Or maybe instead of nuclear it was natural or biological or simply uncontrolled population expansion.