After reading "Daughter of the Empire", I got this idea. The players are wandering about their Western European analogue country, and somewhere gates open. On the other side is an ancient empire of humans or elves or dwarves.
Their culture is very rigid, with castes and lots of slaves. They abhor cowardice, and will not surrender in battle. Prisoners are either made into slaves or sacrificed to their gods, depending upon their relative worth.
The invading Empire is also huge, and has many armies. It may be defeated eventually, but odds are it will at least establish a permanent fortress around the gate.
Over time, the cultures may mix. Local nobles might ally with the Empire, and marry into it. Lots of new armor and weapon styles will appear. New gods may gain influence, to the detriment of the established religions. Maybe a ziggurat will be built, with daily sacrifices to the sun god performed at it's peak.
I just figured this was a great idea. Not sure if it's been done in any established campaign settings, though.