Not only that but he's also signed a "first-look" deal with New Line, giving the studio the first crack at any new property he decides to develop. There's also a telling quote from New Line head Toby Emmerich:
James will make an important movie for New Line and Warner Bros or DC, and that is definitely part of the plan, and a reason for bringing him into the family. He’s the only overall director with a deal here, because we see him as a class of one.
That last bit about DC is worth noting. With only two directors (Zack Snyder and David Ayer) attached to their slate of 10 DC Comics based films, Warner Bros is certainly looking to fill in those slots with capable hands, and some level of authorial voice. Also, what may clinch this is that Shazam is a co-production between Warner Bros and New Line (which WB owns). Lastly, Dwayne Johnson, who was cast as Black Adam in the film, also has a recent history with Wan on the Fast and Furious franchise.
Truthfully, Aquaman or The Flash are just as likely, given the time-frame, but the New Line connection may be our compass rose here.
If James Wan is announced as the Shazam director, don't be too surprised.