I'd like to see D20 Future fleshed out a bit more, as well. In every way. I'll mention specifics, later.
I'd also like to see a *real* D20 Modern/near future campaign setting that gives me something other than D&D Modern, a la Urban Arcana.
I'd like to see a monster book that gives us monsters/foes to use in a way that doesn't scream 'D&D.' You can even use the same stat blocks, but let's alter the flavor a bit to make it something different than "a gate opens up from Greyhawk to modern day earth." Where's the loch ness monster? The Chupacabra? Where would a non-D&D mind flayer come from? What's he/she/it doing? Why?
I want a D20 Modern/Future book that brings real world mythology into the real world, something like Gaiman's American Gods.
I'd like a book (as D20 past presumably well) that does pulp action/mystery within the D20 modern system.
I'd like a book of equipment and weapons that aren't all modern day fire arms. I want a colt single-action Navy revolver! I want to know what I can do with a battery charger. Where are the stats?
I'd like to see a book of maps of every day locales. What's a typical office block map out like? How about a country church? A typical apartment complex? A hotel? Where is a security guard likely to be standing?
I want to see D20 Dark*Matter. Perhaps this most of all.
Similarly, I'd also like to see some of the D20 Future settings fleshed out and given some depth. Dark heart of space, especially. Yum.
I want a book of robots, and how to make and assign CRs to my own.
I want a whole book of space ships.
I want a book of naval vessels from 1600-present. Maybe this could be more than one book.
I want to see D20: Victorian. It's essentially the same time period as D20 Old West, but an entirely different feel. Sidewinder does the Old West very well...now show me the British empire!
That being said, I also want D20: Old West. Not as a replacement for Sidewinder, but rather as a companion.
I want a "Book of Challenges." For real, this time. I want it to detail typical problems that a group in the present day or near future might encounter. In game terms, how do you forge an ID? Fake out a biometric ID? How do you stop a plummeting elevator? What's the average modern day security in a jail cell? Where in the modern world are the sewers big enough to walk in? What's down there?
I want D20 Modern climate books. Jungle, desert, glacial, etc.
I want D20 Modern region books, for most/all progress levels. D20: China, D20: Russia, etc. I think I'm getting into GURPS territory here....
...and that's just what I can come up with off of the top of my head!