I would really like to have someone of importance reply to this.
I know others have mentioned it here on these forums and forums on other sites, but I for one would love to see official reprints of the D&D Rules Cyclopedia and it's companion monster book The Creature Catalog from 1993. Another idea that I am firmly very much behind, is the reprinting of the Gazetteer series of modules (GAZ1-14.)
The sheer size of one hardcover volume containing 14 modules would be impossible, so I would suggest splitting them into maybe 4 books. Each book comes with a map that once you have all 4 books and put the maps together, they form one gi-normus map to hang on the wall!
Many have stated they'd love a reprint of the BECMI-era Known World.