Your gateway to endless inspiration
I eat angst for breakfast, lunch and dinner, this is perfect!!!!
So, we going for a Danny that is like, not Ghost King (or yes, whatever y'all want), but defeated Pariah Dark, has experience with many horrible Ghosts and people, and now, under the Joker's command, can basically destroy Gotham to a single atom, but we all know the Joker wants fun, so he maybe uses Danny to get Jason again, to go back to old times, or maybe for new, he makes him get Damian, whoever It is, will be facing some new stuff.
Wait, what if it was like, by taking an Oath, as you said, it makes it like, kind of a contract, like, Danny is factually under the Joker's command, down to the bone. And like, if we involve the JLD, Constantine (or Zatanna or someone) realizes an abnormal amount of Afterlife magic/ectoplasm stuff in Gotham, tells Batman, and then reads the signals of that magic in more detail, just to realize that they are the same signals of the fucking dude that sealed Pariah Dark away and he apparently is under The Joker's command, by contract?!
Danny moved to Gotham.
Freakshow is touring in Gotham.
Freakshow knows Danny is in Gotham.
Danny knows Freakshow is still after him.
Danny's faith in heroes has been shattered.
Danny turns to the only person powerful enough to run Freakshow out of town, hopefully for good.
Danny turns to the Joker for help.
The Joker is looking for a new punching bag sidekick after Harley Quinn left him.
Danny is just the perfect person to be shaped by the Joker's hands.
Danny becomes the new Joker Junior.