Interesting as this thread is, I'd respectfully suggest people stop identifying spare bits of land behind the lock up garages at the end of their road (including the Dore site).
Topography aside, none are remotely big enough. Those who watched the Netflix car crash Sunderland documentary, will have been impressed by their academy. Built in 2003 at a cost of £13m, it covers 220 acres of which 60 are purely football.
en.m.wikipedia.org
No site of similar size could be built at Dore or in the Don valley.
BTW, Chesea's cost £20m in 2008 and occupies 140 acres.
en.m.wikipedia.org
By way of comparison, a typical pich covers 2 acres and our beloved Lane covers about 12 acres.
Given the piss taking bastards that live on this forum, I now helpfully expect suggestions for a new academy to be built on the Park Square roundabout, Stanage Edge and that 'spare bit' under Tinsley viaduct.