If it were up to me, I'd do a Smokin' Aces style flick with a bunch of DC's best assassins. Bring in Deathstroke that way, along with Will's Deadshot, Bane, Lady Shiva, Talia, etc. Have them all going after the same hit.
As a twist, I'd have the hit be Matches Malone, whose name wouldn't be revealed in any marketing. Have audiences believe it to be the real Matches as the film unfolds, only for the end reveal to show it was indeed Bruce in disguise. Introduce the new DCEU Batman that way.
Hire Guy Ritchie to direct.
Just spitballing here, but could be a fun way to go about the whole thing. I'll write the damn thing myself.