As every time this comes up, and in the hopes of someone reading HN knows someone who knows someone... I'd love to get my hands on the source code of Dark Sun: Shattered Lands and Dark Sun: Wake of the Ravager (SSI Gold Box RPGs), and to a lesser extent Twilight: 2000 (Paragon Software), and Little Big Adventure / Twinsen's Adventure and Twinsen's Oddysey. I'd love to port them to modern systems and/or remaster the art to the extent that is possible. These are games with incredible gameplay but whose art hasn't aged that well.
Also periodic beg to Splash Damage / whoever owns the rights to the content for Enemy Territory to release them, so they can be modded (not the source, but the maps and models). Again, perfect gameplay, would love to see it with modern 3D capabilities.