I think a cool way to expand the D.C. film roster would be too put Batman Beyond as the post credit scene in the Birds of Prey movie. My idea is to have him as the post credit scene where he is in Barbra Gordon’s apartment and as she comes in through the window he is there and says something like “I know what to do” and then kind of allude to a white Martian invasion in the future. This would also be an easy in for the Martian Manhunter. Batman Beyond could also use his future knowledge to guide the universe in what ever way the writers want it to go even if the path seems like a bit of a stretch, similar to Bart Allen in Young Justice.
Had me at Martian Manhunter.