I'm thinking after all this time that has elapsed since the "secret" project came to light that it was probably Memphis.
I have to disagree with you on this being the secret project. If I read right, Umberto was hiring a bunch of people so that could be why there has been a bit of a delay in anything being released to date. Staff transition, all that BS. I think it will be either a mega airport (USA or elsewhere) or an absolute curveball that no one saw coming of something a bit more obscure.
To reply to someone else about KATL, I too hope that FSDT is doing/considering this airport. I tried the IS demo for P3D v3 and frame rates were the worst (under 10 FPS) by a LOOOONG shot out of any mega airport on my system (I own almost all of them) and FAR FAR worse than the killer that is FT's CYYZ (mid 20s). Too bad because I like their newer stuff (ZSPD, KAUS).
Sean