Harley would be cool. All I know of her is the Birds of Prey Box Set. So, I can see her totally being relevant without the Joker, because I've never seen her with him. If they really wanted to cast a Moratorium on the Joker, they open with a scene of a death, maybe even from a distance, and that's the Joker dying, and Harley wants revenge, if you need to justify her without the Joker (Maybe?). A Trilogy would be about 8-10 years, so there's your 10 year Moratorium.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?