I like the evolution idea. In my own personal scifi story, Humanity discovered alien remains and splices the DNA into humans, creating super-intelligent, ultra-powerful soldiers and geniuses. They got bored of being ordered around by a weaker race, began taking over, the creator of the gene treatment forgot to mention he purposely left out a part of the treatment specifically for when this happened, though it turns out the gene he left out was a stabilizer gene that bound their spiced DNA together, making it so that they'd descend into beastly madness rather than degenerate back into humans, as the spliced alien DNA is dominant over human DNA, and self-replicating.
Worst comes to worst and its the Cold War all over again, then the nukes fly. World's destroyed, but humanity survives. Mutated fuckers do too, but they've been reduced to semi-vampyric monstrocities who, upon trying to complete gene treatment, were hit with all sorts of NEW, !!FUN!! genetic mutations. Cue the Resident Evil-style horrific mutations of random types, extra body parts, and odd physical deformities galore.
Humanity bands together, forms clans, gangs, factions, and then the mutated show up and fuck everything up. It gets really fun and weird when the alien-spliced humans accidentally bring the REAL aliens back, who then proceed to enslave a whole shitload of the remaining humans for labor and mobile food dispensers while treating the half-bloods like their utter bitches.
Commence the resistance dedicated to destroying the alien scourge, Ach'riminos which means 'The Suffered Ones/They who have suffered' in my language. There's a whole complex story on how the original species split into many races and factions, all of which dominated different worlds, like the 'gods'/lords of Stargate. Earth was the most coveted of those worlds, due to its WATER, resources and MASSIVE amount of free slave labor, and there was a huge war over who got to dominate Earth. Eventually a rogue faction called the Ach'riminos decided to sever all links to the slave-planets by destroying the technology used to travel there, leaving the few Ach'remoc stuck on Earth to die off after being revered as gods, or feared and hunted as monsters of the night, while Humanity flourished with their new found freedom - of course, thanks to the progress their masters brought into our world.
Etc etc etc.