The other one that keeps coming into my head is the
other half of the NY Long Path - the half that isn't blazed, because it's a route rather than a trail. A good description of the route as it it existed eighty years ago can be found starting on page 26 of
http://www.nynjtc.org/files/documents/LBP.pdf.
With the state acquisition of the Finch Pruyn lands, it would be feasible to do virtually the whole route as described without trespassing. (When I read the 1930s-vintage description of the route, it seems charmingly naive, "you're trespassing here, so don't build any fires or make camp," rather than 'PRIVATE! KEEP OUT!")
There are only a couple of reroutes that would be absolutely required (avoiding Mason Hill in favor of following the East Stony Creek up from Northville; something to get around the private land near Crane Mountain; finding a bushwhack route up Gore from the southwest), and some open questions (can the dam maintenance road, disused for a century, be followed from Boreas Bridge to complete a route into Boreas Ponds, or should the modern route detour east to follow the Gulf Brook tote road? Is the tote road from Casey Brook to the Elk Lake-Marcy trail a passable route?) The missing section is easy to reconstruct from the mileage table. It descends from Four Corners to the west as far as Lake Colden, heads north through Avalanche Pass and finally picks up the Van Hoevenberg trail to the Adirondak Loj.
Combining the route as described with a thru-hike of the blazed Long Path would make for about a five-hundred-mile Big Hike, including some dozens of miles of unmarked, abandoned haul roads and maybe 10-15 miles of outright bushwhack. That hike would take in the best of Harriman, the Shawangunk ridge, the Catskills (including the Burroughs Range, half of the Devil's Path, and the northern Escarpment), the Schoharie valley, the Helderberg escarpment, a TON of roadwalking to get across the Mohawk valley (sorry! No workaround here! At least it's pleasant farm country with good views, like the Cumberland Valley was when the AT roadwalked through it), and then the very best of the Adirondacks: East Stony Creek, Crane Mountain, Gore, the Hudson Gorge, the Boreas Ponds, Panther Gorge, and a circuit of the Adirondack Great Range, before finally going through the spectacular Avalanche Pass out to Lake Clear of Heart.
Logistics for it would involve a lot of homework. There are sections where places either to camp or to pay for lodging are pretty sparse. Finding the route would mean carrying 1895 topo maps as well as modern ones.
About 5-6 years from now, if my body cooperates, and I can find a suitable crew (I would not attempt such a crazy stunt solo),I might just give it at try.