The thought occurs to me that there has been a multitude of examples in literature of the hero being a man (occasionally a woman, but I think the disparity greatly favours men) from "our modern world" - whenever that is - who is transplanted to a fantasy world.
It never seems to be the villain who is the cross-world voyager.
Oh, there's occasionally a lesser bad-guy who is such, usually weak, venal, or tricked into assisting the true villain, but there's not (to my knowledge) any example where the Dark Lord(tm) is such a traveler.
So, depending on your groups style, you could spice things up a bit in your fantasy game by having the new and terrible threat be from a modern (or futuristic) world. You'd have to make such a villain a competent, lucky, and powerful as the heroic types of the stories, otherwise they wouldn't survive. Let's face it, if dropped into a medieval fantasy world, most of us would be dead in short order. This has to be someone who is able to survive, prosper, and be a credible threat.
Motive becomes something of an issue, however. In the literature, the hero usually picks the correct side by pure luck (and/or incredible prejudice), if he does pick the wrong side, he's usually made aware of this pretty early on and gets to go on a revenge kick as well as protect the good guys. This could work for a villain, but you'd still have to work out why he's evil.
The villain has been converted somehow. (Been made a member of the undead, "seen the light" (dark?) and converted to an evil god, etc)
The villain is trying to make things better, casting down unfair monarchies and re-building the world in a more "sensible" fashion.
The villain is out for blood. (The fantasy world's wizards' guild refused to help him get home/refused to believe him and so he's going to smash them all, despite this nonsense about cosmic calamity...)
The villain senses opportunity... (Just because he's a cubicle-monkey back home doesn't mean he's going to be one here).
The villain doesn't want to go home... yet some heroic group are trying to make him. (This ties in well with the above, but on its own is a bit weak).
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