I'd love to see DnD'd Discworld.
If you think about it, bulk of the work is already done, and transitions would be relatively easy, ast least to 3.X/PF. Their wizards as well... wizards (their spells are probably the only ones outside of official DnD franchises that work like the ones in DnD), witches as druids...
It should work with 4e as well - the "fleeing spells" being dailies/encounters. Obviously, Discworld wizards use a lot of ritual casting (both teleportation and Speak with Death* are described as ritual, cast by a group of wizards).
*not to be confused with speak with dead
It could even implement some meta-gaming from DM - for example allowing any RAW exploits - once (as described with L-space computing (can't recall exact book)). L-space seems to me to describe 4e Multi-universe completely - rules changing constantly as mortals fiddle with them and new synergies are found.
It's IMO a resource far too untapped as far as RPG's go.
edit:[MENTION=19675]Dannyalcatraz[/MENTION] - oooh man. Yeah, I'd love to see Pathfinderised Dark Sun. Licensed, or a cheap rip off, I honestly wouldn't care (OK, I actually would, because that would make hardcore DS fans unhappy, which would mean a strange direction for the setting. But still.).