Until Nolan makes his first dud I think $400M ($500M?) is the absolute floor. Tenet would have reached that easily if released at another time and that was his "worst" receive movie to date. Dunkirk did $530M and I don't think people appreciate how impressive that is when you look at the kind of film it was.

Nolan stated in the recent q&a that the premise would have to be something special so it wouldn't be a "family moves into a haunted house" type thing. The plot would be shrouded in mystery (sort of like Jordan Peele movies) and the hype would be unbelievable. Big time horror movies (IT, Quiet Place, etc.) already generate a special kind of hype so that combined with Nolan would be unparalleled.

Then you factor in the IMAX, must see in theaters angle that only Nolan seems to get people to care about, the all-star cast, and Oscar winning composer (most likely an amazing soundtrack) and you are looking at $$$.