It was Leonard Nimoy in DIE HARD that brought me there...
These are pretty well circulated at this point, but my favorite is still Fritz Lang doing a hypothetical 2001: A Space Odyssey. Having that movie done through the lens of German Expressionism? It'd be fantastic. Its such a fascinating period of film, if only because you get to see people taking the technology at the time to its logical conclusion.
Fuck me! I would kill to see Klaus Kinski as the Nihlist. I'm not sure FZ would make a good Dude, however. Zappa was way too loquacious and cerebral to be the dude, and he was always adamant about not doing drugs. I know its acting, but c'mon, doesn't fit his personality. Jeff Bridges, like the Dude himself whom he was playing, was the right man in the right place at the right time.