Just a couple of ideas that have me by the choke hold right now
Danny is Jason's older brother who died and got reincarnated. After circumstances force him to Gotham, he finds out that his beloved little brother is now a crime lord
As all the others leave the league of assassins, Danny is left behind to be molded by Ra's Al Ghul. One day Ra's decides to unleash his prized assassin on Gotham
Danny moves to Gotham and starts helping out at a local pet shelter (mainly so cujo can make friends). It is there where he runs into one Damian Wayne who is setting off his must protect senses
Danny becomes a back door engineer. He fixes things for cheap no questions asked. No one can beat him when it comes to using whatever you have laying around to build something from scratch. That's why he's Red Hood's personal mechanic
Danny is a clone of Batman and Superman. Superman is grateful that he can finally do right by his clone. Conner is exceedingly jealous of Danny's treatment. Batman is contemplating taking his friend to court for custody. And Danny is just vibing because now he has two powerful dads who won't let the giw get their hands on him.
And the final one that is really rotating around in my head:
Danny is Batman's informant. He can get any info the bat needs in record time. Batman is the only one who knows about Danny and his abilities so whenever anyone else catches a glimpse of him, they think it's Bruce's secret love child. Damian is not happy about this.