While The Batman seemed to be on the right track a couple of months after Ben Affleck officially announced that he would direct, it all derailed when the actor stepped down. After some intense negotiation, Warner Bros. hired Matt Reeves (Cloverfield, Dawn of the Planet of the Apes), who started everything from scratch. Although Affleck had said on a couple of occasions that the script for The Batman was almost complete, Reeves decided that a brand new script was in order, which meant that the original 2018 release date couldn’t be met (especially since War of the Planet of the Apes is coming out this July).
For that matter, although supervillain Deathstroke (Joe Manganiello) was previously announced as a main antagonist, it now appears that he won't be in the movie at all (Manganiello has fueled these rumors during interviews).
However, a new rumor from the Reddit DCEU Leaks community says that Deathstroke will indeed appear in the movie. The source added that, even though Matt Reeves probably hasn't begun to work on the project (being busy with War of the Planet of the Apes), Warner Bros. executives know that he wants to use multiple villains:
No work has been commissioned on this movie. Apparently, Matt Reeves has yet to even start on this, and has to fulfill his commitment on War for the Planet of the Apes before working with Ben Affleck. Although there have been rumors of the script being thrown out, there have been rumors around the executives that Matt Reeves wants to incorporate many Batman villains. These villains will include Deathstroke, Joker, and Scarecrow. The studio has already been working on Deathstroke (obviously), and has mockups of a potential new Scarecrow.
While it did seem weird that Deathstroke had been completely eliminated (despite the fans' reaction to his test footage), it wouldn’t be surprising to see the Joker. Jared Leto recently teased this possibility.
As far as Scarecrow is concerned, anything goes! But we hope that this version of the supervillain will be more terrifying than the one we saw in Batman Begins.