At this point only speculation could point to what other DC villains could have made appearances in The Batman.
However, this report could also point to a number of classic Batman villains making their DCEU debut.
Joker’s inclusion on the other hand is not surprising as he is the classic Batman villain.
With Harley Quinn out on the streets with Joker, she could have been a likely candidate as well, along with potentially her Gotham City Sirens cast mates.