I like Assault on Arkham idea, but I hope they use that more with SS2. With Diablo out, hope we get some more metas and large guys like King Shark and Killer Frost.
I would like Joker to be the main bad for the Harley film for a few reasons. Leto was knocked off too early and he really was great. Ayer sold him on the SS script, I can only imagine he'd be able to sell him again for Sirens. Also, seeing as it is initially a Harley solo, Joker makes the most sense. Though his story would be interesting to try and take since Harley is now considered an anti-hero. I think one thing could be Joker taking over Arkham, maybe using Ivy against Harley. Or, and this I'd prefer, is a Death of the Family style story. This film could be very much an Ant-Man of the DCEU, fun and great, and no need to emphasize it as some grand scheme or something.
The featurette was really fun too. I won't lie, I was bummed not to see some parts like the confrontation at the Lambo, or when walks away after the Enchantress battle.