The question is more difficult than it first seems. After all, we have almost a hundred years of fantasy, ranging from pulp to modern epics.
For me, I'd love to see the whole Eternal Champion thing boiled down in D20. Not just Elric or Hawkmoon, but a sourcebook for each with a book on the Eternal Champion concept itself and how the balance and the sword, the champion and the companion, work through different times, eras, and each other.
Next up, I'd like to see that series by C.S. Friendman, one of the books is Black Sun Rising I believe.
I loved the variosu books by Weis and Hickamn about the Death Gate series but hated the end. Seemed very "Oh. That's it? Okay."
The Raymond Feist Magician series with a timeline that allow the GM to determine when and where he'd place the campaign.
Others have already mentioned some of the more often hit upon subjects.
Conan.
Fafrd & Gray Mouser.
Thieves World (They just came out with a new novel on Sanctuary. Any good?)
(What's the name of the other race? Derynia? I know Fudge has a book coming out about 'em...)
Morgainie (The C.J. Cherryh books.) Be good for mixing up some sci-fi with fantasy. More towards the latter than former.
The Horelord/Dragonlord/etc.. series. Pretty interesting stuff when I first read it all those eyars ago...
I'm sure there are hundreds more we could and will think of.