Here's my list:
Rifts d20: I'm doing my own (for home use, so Kevin Siembieda and his giant ego can bite me), but I'd much rather play Rifts D20 than constantly convert it!
Willow/Shadow Wars D20: These books simply don't get enough praise. I think it would be a blast to game in this world.
Marvel Universe for M&M
Image Universe for M&M (mostly the Wildstorm and Top Cow Studios characters)
Warhammer 40K D20: It'll never happen, but the idea of fighting wave after wave of Chaos Space Marines in a system that is fun would be great.
I'd love to see a setting based on the works of Sean Stewart. A world after a magical apocalypse. Sort of like Rifts, but magic is more subtle. Minotaurs are born from nightmares. Cool stuff.
Final Fantasy D20: Focusing mostly on the fantasy/steampunk games rather than FF7, 8, and 10.
Another Star Wars book covering the species that were left out of the UAA as well as all the creatures.
The Fifth Element D20: Expand upon the movie. This setting would be great for tongue-in-cheek sci-fi.
Doom/Quake D20: For violence, mayhem, and horror nothing beats these games. It'd make for some great sci-fi/survival horror games.
Planescape D20: My all time favorite 2E setting.
Spelljammer D20: It was weird, but that's what I loved about it.
Dark Tower D20: Stephen King's fantasy masterpiece (hopefully he'll be able to finish it). This setting would be wild.
That's about it.
Kane