Before the SnyderVerse fell apart and The Batman was a thing, and then even that was semi-superseded by the new DCU, Ben Affleck was to star, direct, and co-write with Geoff Johns.
It would have been a new DCEU entry, and would have followed on from the events of Justice League.
All anyone really knew is that Joe Manganiello’s Slade Wilson/Deathstroke would arrive in Gotham City to take down Batman, as glimpsed in the post-credits scene in Zack Snyder’s Justice League, where Deathstroke arrived on Luthor’s yacht.

After a while of back-and-forth, mostly from Warner Bros. Customary tinkering and inability to sort itself out regarding DC movies, Affleck then stepped down from the director’s chair, aiming simply to star in the film.
Then tragedy forced Snyder out of the DCEU, Justice League flopped on release, and the DCEU stuttered on before it was halted.
Matt Reeves decided to reboot Batman, Affleck exited, and eventually the new DCU came along to replace even that.
What a mess!
Now, a conversation on Reddit, posted by ComingSoon.net (via Yahoo) with a pre-vis artist who worked ont he project has confirmed a few things.
Deathstroke was the main villain, but as the plot heavily featured Arkham Asylum, there would have been appearances from several of Batman’s rogues gallery, several in more than cameo form.
Particularly notable was Mr Freeze, with the artist proclaiming they were his favorite scenes, and would have been “beautiful and haunting”.

Well, now we will likely never see them. A new DCU Batman movie remains in development, with The Batman Part II commencing production in the UK in a few weeks.