I would have loved to have seen D20 Modern campaign guides for all of the old TSR setting -- Top Secret, Boot Hill, Gamma World, Gangbusters, Star Frontiers... Yes, we got hints here and there in the supplements that were produced, but I would have bought well-done hardcovers.
I would like all of the Babylon 5 Wars expanded universe updated for use in Mongoose's RPGs (either D20 or Traveller). More League of Non-Aligned Worlds, the Centauri-Orieni, etc.
I would like a book talking about the origins and evolution of iconic monsters from D&D (not in-game...the inspiration for the original design and why they changed over the various editions).
I would like a second volume to Monte Cook's D20 World of Darkness. I don't even really care what is in it, as long as it is more of his take on the setting.
I would like to see Transhuman Space from GURPS 3E as a Mongoose Traveller setting. I always liked the setting, but liked the Traveller rules better than GURPS.
I would like hardcopy Dragon magazine again.
I want plastic, pre-painted miniatures at 1970s prices.
I want season 2 of Firefly.
I want an updated Stargate RPG that encoporates the whole expanded universe in either Traveller, D20 or Cortex rules.