Prompt: Danny Fenton Todd
Jason is just chilling in a safe house at home when an ominous swirling green portal opens in the middle of his living room and several beings with giant eyeballs for heads pull a trussed up teen through.
Freaky eyeball: "Congratulations, revenant, you have been chosen to be the abom- I mean... the halfa's new guardian."
Jason: the fuck did he just call this kid?!
The eyeballs then promptly leave, the teen still trussed up in the middle of his living room. Jason unties the kid and sets about interrogating the kid because wtf. Jason finds out:
Kid's name is Danny
He's 15
He has no family or home to return to
He is a half ghost and has all the ghost powers that come with that.
Jason had an older brother once, his name was Danny, he was 15 when he died. Jason looks at Danny: black hair, blue eyes, simmering rage, gaunt corpse-like features. Yeah, this could work.
"Roy! I need a favor. I need you to hire someone to dig up my brother's coffin and transport it out of Gotham and, this is important, it needs to be super obvious until it's out of Gotham. Then Batman needs to lose all trace. Got it? Good. Okay kid, you need to turn invisible and stay invisible while I go steal a plane."
Cut to Batfam PoV, where Jason suddenly leaves town with no explanation, then the next day his older brother's corpse gets stolen ("Jason had an older brother?!") which Bruce fears is a sign their IDs have been compromised. They try to figure out what's going on but the corpse trail goes cold outside Gotham and Jason isn't answering any calls. He's all over Eurasia, fighting all kinds of people, even ending up in a few LoA bases. He finally stops at Nanda Parbat (to intro Danny to the remnants of the All Caste, but the batfam doesn't know that) before stealing yet another plane and heading back to Gotham.
"Hey everyone, this is my big brother Danny. No, we will not be taking questions at this time. C'mon, Danny, I'm exhausted, let's go home."
I'm imagining Danny just getting dragged along, not really understanding anything that's going on, but his core trusts this guy, so he'll go along with things until he gets his head on straight.
My idea is Jason is explaining some things during the travel time at least. The flights over the Atlantic alone are hours of uninterrupted explanation time. He should at the very least be explaining Danny's new backstory and anything he would be expected to explain to his brother upon being revived.
Now, DP and DC being 2 different universes and Danny having no idea who Batman (or Superman or Wonder Woman...) are and not bothering to ask could definitely leave Danny even more confused.
I've always preferred separated universes anyway because a, at least by appearance, unmentored teenage hero should draw attention in dc, especially post justice league founding, unless you give a really good explanation.
It absolutely should! But personally I'm torn for narrative reasons. If they're different universes there is no Danny Fenton for the Bats to eventually dig up. There's no billionaire Vlad Masters out there searching for his missing godson, putting up wanted posters and Danny's face on every milk carton (he doesn't care if they don't do that anymore, he's paying them to do it!)
On the one hand if you don't want that complication that's fine, there's plenty of drama to go around as is (especially if the reason Danny has no home to go to is his whole universe got blown up by the GIW). But on the other hand there's the drama of one (or more) of the Bats seeing their new brother's face on a missing poster and knowing that now they have to go tell Jason his brother isn't actually his brother after all.
Separate universes or not, the first complication that comes to mind is that there will be DNA tests. Even if Jason says he already did them, Bruce at minimum is paranoid enough to do them again. Tim might also. If only to have a thorough medical file on the Batcomputer in case of emergency.
Anyway, there will be DNA tests and I'm pretty sure a total lack of genetic relation to Jason, Catherine, Shelia, or Willis would be a rather notable discrepancy.
Considering Jason was kinda just dropped off with Willis as a baby, they could always argue Danny had been as well, and turns out, Willis wasn't the father after all, who would have thought. Sucks for him.
Oh, the point wasn't that there was no explanation. It's more like nanenna was talking about with how putting DC in a separate universe removes certain DP elements that could otherwise be used to create conflict in the story.
For narrative purposes, assuming the end goal of this story would be for the rest of the Bats to learn the truth, there needs to be 'complications.' Cracks in the story that the Bats can question. Clues.
And even if each crack on it's own has an explanation, just having a lot of them pile up over time will steadily weaken the cover story. Until a shift in perspective allows all those ill fitting pieces to snap into place, the lies fall away, and the big picture resolve into truth.
And I figured one of the first cracks to come up would be the DNA mismatch.
Ooooh! That is a very good point! And it's definitely something I was also thinking about but hadn't really touched on because this idea is forming as we go, originally I just wanted to share the base seed because there's so many different ways this could go.
The first crack is actually the inciting incident. Who dug up Danny Todd's coffin? Jason comes back with someone he claims is Danny and never explains it. If the Bats were able to track Jason's rampage around Eurasia there wouldn't seem to be anything connecting the various places and people he trashed. Whoever is the current Head of the Demon called up Bruce upset one of his kids invaded a couple of their bases (Bruce reminds them he has no control of Jason and hasn't since they threw him in a Pit).
The DNA is the second crack, this kid who's DNA doesn't match Jason at all (if it doesn't just come back nonsense). Oh sure Jason has the whole cheating thing to fall back on, but that doesn't explain why Danny doesn't have a Gotham accent at all. Or a hundred other little things that are all going to add up.
I love it if by the time they finally
Figure out everything all the lies and full of shit and they actually get a machine that works in Danny’s blood they find out that Danny is in fact, Danny Todd 
That Danny’s body was stolen experimented de aged and a lot of other shit, and then given to the Fenton to be raised 
Alternatively if you're doing alternate universes Danny could just br DP's Danny, or atleast have enough DNA in common to come up as a half sib even if not an exact match for Danny.
Could very well be why Jason was chosen.
I kinda want it to be same universe, and Danny is actually for real related to Jason, but he's actually his great uncle or something. Something like, there was a Clockwork time travel adventure incident, Danny got sent back in time along with Dan (forced to do community service because keeping a prisoner in eternal solitary confinement is bad actually and Walker would be hopeless), and then Dan got hitched and settled down in the past (MCU Captain America-style). Maybe Danny matchmade him with someone, either entirely by accident and without noticing he had done so, or on purpose because he saw an opportunity to do something funny (or something he expected to annoy Dan) and took it.
Oh now that's a new angle! Dan is a complete copy of Danny, right down to the DNA. Maybe it hasn't happened in the present yet, but Dan getting sent to the past and settling down, maybe something a bit closer related because your DNA tends to fade into the background noise in only a few generations so: Dan is/was Willis's dad. Danny has no idea that did/will happen so he's not prepared to come up somewhat related to Jason. Neither is Jason. They're both baffled.
