As far as which character would best fit in an R-rated DCEU film, a clear choice would be Lobo, which is currently in development.
To a certain extent, Warner Bros. has already released R-rated films, having delivered an R-rated extended cut of Batman V Superman: Dawn of Justice.
Clearly, if Warner Bros. is serious about making an R-rated adventure within the DCEU, the studio won’t be lacking for options.
As far as the rest of the DCEU is concerned, other R-rated contenders could include Aquaman, given James Wan’s background in horror, as well as Black Adam, if that’s how Dwayne Johnson envisions the film.