Logically, I can see the heads of Fox and IMAX practically begging Cameron to take summer 2018 and run with it. Anything else would undercut the film's BO potential severely.
Or maybe this is the one time Cameron's ego eats him alive, he says FUCK IT, and gets run over by VIII because the vast majority of IMAX/3D screens are taken.
His response would likely be "Why? I did it already. With REAL cars. Neener, neener."
(I may have made that last part up.)
He'd probably be more interested in seeing if Vin or The Rock fancied a vacation to the Namibian desert. If you know what I mean.