Re: What-If (WhIf) Setting Resources
Well Turtledove is known as the father of Alternate History. One that I think could work very well is yet another retelling of WWII but using magic instead of technolgoy.
One for the more cerebral is Days of Rice and Salt by Kim Stanley Robinson where all western civilisation is destroyed by the Black Death and the Asian, African and Indian (both India and Native American) cultures develop. This would be a particularly good one to dump players in for, "you wake up one morning . . ."
Another interesting one could be the John Birmingham series where a futuristic fleet goes (yet again!) back to WWII. Similar to the film, 'The Final Countdown' but history changes when the ships stay there.
Also, points to OzBeastRabbin (who's name comes from Dune)! In the last clip, he uses footage from several films including Memphis Belle (I think), Band of Brothers, War of the Worlds, Independance Day, as well as stock footage of WWII with the music from Aliens. For reference though, the aliens that attack in those stories are actually 4' tall lizardoids, not giant death machines and their fighters used missiles not 'green energy' but full marks for what you did. It was awesome.