Would it be completely cheating to pick ShadowRun? Cyberpunk meets fantasy in a shiny pre-packaged format.
Or should I be thinking more along the lines of cross-over shows, where Jessica Fletcher ends up solving Magnum PI's crimes, or Sam and Dean save Mulder and Scully from fairy-aliens. Or something.
Hmmm.... If you wanted to combine two D&D games, you could always pitch both parties into the misted realm of Ravenloft. You'd need both sets of players to be willing and understanding of the setting in order to have real fun with it.
But there are so many more exciting games you could bring together.
I loved the World of Darkness game played at uni: same players, different characters in Werewolf, Vampire and Hunter. Different agendas, of course, but it meant we got a better look at the bigger picture as the world ended.
Actually, back to ShadowRun and the first time I played it the GM really wanted to run the Renraku Archology adventure. He ran it with us, his main group, and then after my character had escaped with an NPC, he used our desire to get back in to rescue the others as the method to get a group of his old uni friends in too - earning me xp in my absence, because the NPC & I were being so cautious and suspicious his other friends nearly killed us!
The game had many players and each session would be a run based on whoever was free. It gradually fell to a few key players, but the idea of multiple players not always present still appeals. I'd like to run my own ShadowRun or Hunter game that way: I have ideas for both. But I'd also like to take it even further: if there were two groups running in the same setting and the same time, with their GM's in contact with each other so the actions of one group could be seen by the other. I'd love for the players to not necessarily know the games are affecting each other, but I'd love to do one massive session where the two groups are sent on the same run by different fences. At least to begin with, they'd need to be in separate rooms but they might end up working together and then the GM's could combine forces...
I'd love to take part in that!