Zelda 64, a prototype from before Kid Link was introduced, with the old Hyrule Castle dungeon, the Turtle Rock styled Dodongo's Cavern (can confirm it is, just much earlier) and fully modelled Castle Town.
I know I would die before I ever see that leak, assuming it still exists, but dang if I don't want to.
Or files on the early development of Final Fantasy IX. There were some small, but major, changes late in development, but even early concept art shows a bit of a different game with a job system, using earlier versions of Zidane and Garnet, and a story about mages and dragons. Don't get me wrong, I adore the IX we got, but I'm still curious about that earlier form of it.
darkSector as it was originally shown on 6th Gen hardware. It sort of became Warframe, but I loved that game and picked it up knowing it was originally a UT sequel, and I'd love to see the earlier form that D3 passed on publishing in order to make it more realistic and gritty.
Alternatively, the hi-res background renders that were lost. I know they're likely forever gone, but one can hope.

Lastly, a Halo Mac OpenGL prototype. Like what was shown at the Keynote. Please? I would adore it!