The mother of all failed projects has to be Big Thunder Mesa/Western River Expedition, the epic E-ticket dark ride that was slated to go into Frontierland (and may have had other rides attached). Lots of concept art and descriptions of what could've been out there.
If you find the right rabbit hole with Google, there's a lot of concept art/descriptions of David Copperfield's Magic Underground, a show-heavy-restaurant that even broke ground at MGM Studios. Same era as a Noah's Ark musical penned by Andrew Lloyd Weber, but I've never heard of so much as a leaked track from that.
The late Jim Korkus had a few books on Disney history, one of which detailed the history of Pleasure Island. The unbuilt concepts that stayed with me were Madison's Dive, a nautical themed bar not unlike the Adventurers Club, and (admittedly dumb) the 24-hour TGI Fridays, which would've been a lot more unique and impressive in 1989. Later, after PI shut down, a couple cool restaurants were teased for "Hyperion Wharf" (the new name for the area). AeroBlue looked amazing in the concept art--WW II plane hanging above the dining room with a saxophonist--there was a North Pole one as well I think, and a Landry's aquarium restaurant was slated for the old Virgin store.
If you're into this sort of thing, I'd recommend Defunctland and Poseidon Entertainment on YouTube. Not all they do but a lot of videos if you go back through their archives.