What if the cataclysm that shattered the modern world was the return of magic? It had been active during their version of the ancient and medieval time periods, but slowly went away. People started to rely on technology and robots instead of sorcery and monsters. Then, when magic was all but forgotten, it returned with a vengeance. Random people (latent sorcerers) started exploding, digital systems went offline, and governments rushed to create sorcerer-soldiers to fight in one final war. It did not end well.
Centuries later, dinosaurs roam the earth, coexisting with humans and their various magically mutated offshoots. Sorcery and gunpowder are both utilized equally, and science has been largely abandoned in the pursuit of the ever-changing logic of magic.

(What I'm imagining here is train-robbers slinging spells and riding raptors.) I think this idea is pretty cool.