That would get me to open my wallet.THE WORLDS OF DUNGEONS & DRAGONS - a hardcover with chapters giving overviews of every campaign setting ever published by TSR or Wizards of the Coast, with world maps and basic info. This would include Tekumel (maybe), Blackmoor, Greyhawk, Dragonlance, the Forgotten Realms, Planescape, Spelljammer, Birthright, Jakandor, Dark Sun, Eberron, Mystara, and a few others I'm undoubtedly missing. I'd of course love to see non-TSR/WotC settings included, but you've got to draw the line somewhere. The book would also include appendices detailing every product every made for each setting.
The Classic Rules Cyclopedia back in print. The only complete-in-one book version of D&D ever made.
A computer RPG sequel to Planescape: Torment. More than anything.
...failing that, Planescape 4e. Including stats and bios for all the major Torment characters, and the return of the original factions (even if not based in Sigil).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.