"All that the sea wolves have taken, all that the war birds of the skies have claimed shall return to your shores and seed strife amongst your kingdoms!" Those were the last words of the gods upon their defeat. Of course, if they truly had any power at this point, they could have stopped the collected armies from killing them.
But in some other world, it took hold. Strange objects have appeared across the lands ever since. Odder yet is strange people that also come to out towns and kingdoms. They who know not even the basic fundamentals of hefting a sword or using a tinder box like any child, yet identify the strange metal wagons as a "'68 Camero" and the odd pointless spears as the work of the great armorer "Kalashnikov".
People like you, I take from the odd clothes you wear and that strange box you were holding to your ear.
***
The basic idea is a fairly standard fantasy setting, complete with Lamia, Gnolls, Orcs etc. (Though perhaps a bit friendlier than the D&D standards - all races at one point were united.) Into this is injected various items lost from the 20th century. Warships torpedoed in World War One, Victory ship merchantmen laden with M4 Shermans, AN-26s full of RPGs and PKM machine-guns, perhaps even the occasional armed MiG from Yugoslavia.
Most of this stuff comes off as magical or at least well beyond any current technology by centuries, and quite mysterious to the average inhabitant. However, occasionally people from the 20th century show up as well, and they do know what at least some of this stuff is. As you would imagine, these people are key to unlocking great secrets and highly sought after by kings, mercenaries, and revolutionaries for the ability to introduce powerful technology to their cause.
I'm still debating as to what extent the 20th century has a presence here. On one hand, it might be nice to keep the players as relatively special and amongst the rare "visitors". On the other - entire lost battalions from WWII and the occasional platoon of spetsnaz means you see a lot more equipment, and some modern conflicts bleeding into the medieval landscape - WWI still being fought amongst the Roman age legions of gnolls and nomadic lamia aiding a Lawrence of Arabia like figure...
Possibly mirroring the usual Mage/Fighter/Thief paradigm, we might have loose character classes of what type of foreign equipment you're familiar with. Soldiers obviously know the most about the weapons and how to use them, but other might know more about maintaining vehicles, or synthesizing chemicals to keep them moving, or general engineering projects to help out towns.
Somehow, its only now that I realize this might be taken as "Armies of Darkness" or "A Connecticut Yankee in King Arthur's Court" just on a very large scale. Still sounds like fun though.