Stjepan posted a bunch of concept art for Harleen volumes 2 and 3 over on Twitter.

He also posted this in answer to a question of future Harleen volumes:

not confirmed yet, but i have my outlines
Not sure why DC are dragging their heels here. The hardcover over on Amazon is currently #1 for superhero comics and #5 for all comics once Kindle editions are removed, and a lot of the heavy lifting for the contract ought to have been done when Harleen 1–3 were negotiated. Unless Sejic's proposal to do "Isley" before "Harley and Joker" is causing trouble here.