Equestria has resources not found in any other dimension, so of course an alcoholic, mean-spirited scientist would get in on that and drag his grandson along for the ride.
When the Winchesters have nowhere else to go, Dean asks for Castiels help and they both get transported to a magical realm of friendship love and ponies. Ponies, seriously. Dean is back in hell, no worse than hell. He's in Equestria.
He's the God Emperor of Mankind, and she's Princess Celestia of Equestria. With these two dating, you can expect almost anything to happen. Hopefully Equestria survives their relationship in one piece...
Darth Vader, one of the most insidious villains in the entire galaxy, ends up in Equestria of all places. With no support from the Empire, this Sith Lord needs to find a way off the planet. Or he could take over it. Either one works.