Before anyone posts on this, I'm aware that two, three if you count Mazes and Monsters (*gags loudly*), Dungeons and Dragons films have been made and that they suck, badly.
As a side note, FOX had a, if you can believe it, Vampire the Masquerade TV show back in 1996, but due to plot confusion to non-gamers and other factors, the show ended after only eight episodes. Ouch. (Sci-Fi shows it every so often, or you can buy it on DVD.)
However, considering all the RPGs out there, I can easily see another film that is based on one of them coming to light some time later. However, instead of non-gaming script writers, the publisher's writers should be the ones that write the script for such a thing, since they know how to present their work best.
Of all the companies I know, White Wolf's World of Darkness series, old or new, would be my top pick. Werewolf, my personal favorite, Hunter, Mage, and Geist are the four I'd want to see.
Potential others -
Midnight: Second Edition - Think Lord of The Rings, except this story begins after Sauron wins in the end.
Unknown Armies - In this game, your characters deal with the weirdness and realization of magic in a modern day setting.
Call of Cthulhu - Need I say it? H.P. Lovecraft's monsters and stories started horror fiction.
Agree? Disagree? Have another idea? Say so.