Jason, being a semi-canonic common hallucination in the family after his death, could lead to the stupidest AU ever.
Imagine everyone seeing him — Bruce, half of the time, Dick non-stop, Tim more often than not, and eventually even Alfred starts seeing little boy's silhouette in the corner of his eye, but he never admits it, because someone needs to stay sane in this family.
It is a lot like real-life cases when cult families start to see collective hallucination, and it somehow syncronises in their minds, so they hear and see the same things, you know?
So, yeah, everyone sees Jaybin around.
Everyone but Damian. Damian is a normal one. He also knows his Akhi is alive and well, so whatever. And it takes him some time to figure out that his family is bat-shit insane, but when he does, he decides to use it on his advantage.
Damian, calling Jason: Akhi, you should visit me. It is getting awfully boring here.
Jason, frowning: You know I can't. They think I am dead, and I can't risk my plan, especially now, when Red Hood is gaining-
Damian: We will pretend you are a hallucination.
Jason: ...What?
Damian: So, there is a plan...
So, a few days after this call, Jason arrives at the Wayne Manor. He still thinks his brother's plan sucks, but gaslighting is one of his many talents, so surely, they will figure something out. He can lie his way through this meeting.
Expect, he doesn't even need to lie. His family is actually insane.
Bruce, bumping in Jason:
Jason, staring back: Uh-
Bruce: Wow. You look so grown-up. And we look so alike. Nice one, brain.
Jason: ?..
Tim, leaving his room: Hi, B, hi- Oh, damn. Hi, Jaybin. Nice leather jacket.
Bruce: Right? I guess his ghost just grows up with us now.
Jason: ????
Alfred, nodding along, out of nowhere: Master Dick will hate it. He looks taller now.
All of them: (peacefully leave the room)
Jason: What. The. Fuck.
Jason waits for the moment of clarity to happen as he chats with Damian in the kitchen, but... nothing changes. They really, really think he is a hallucination. So... he starts hanging out around more. Both because Damian is getting angsty, and because it is kinda... amusing.
Tim, stuck on the same case for a few nights, non-stop: Oh, it is really just me and you in this, Jason.
Jason, playing Mario Cart on the table by his side: Maybe take a nap, dude.
Tim: No, I need to figure out this case with-
Jason, rolling his eyes: Red Hood had already dealt with it. Go to sleep.
Tim: ...You are such a good self-care kind of hallucination.
Jason: ...
Damian: Your bets, when will they realise that you are a real person?
Jason: At this point, I am not sure that they will, even if I start screaming that I am real.
Damian: Fair. I bet a year would do.
Jason: ...A year and a half.
Dick visits the Manor. He cooes at Jason, muttering something about "of course, he would have grown up in a punk," and Jason almost breaks his role to hit him on the head.
Jason, arms folded on his chest: You know, you need serious help, dad.
Bruce, blinking at him slowly: Probably. You know what else I need?
Jason: Sleep? Retirement? To stop adopting strays? The list is endless, man.
Bruce: ...Coffee. I need more coffee.
Jason, groaning: What the fuck!!!
Alfred figures out that Jason is real, eventually. Solely because he catches him sneaking a few extra cookies, and hallucinations are not supposed to eat. He plays along with him and Damian until the very end, anyway.
(Damian ends up winning the bet because Jason loses it once and pushes Bruce down the stairs, when he starts reciting some precautionary tale about him. Everyone is flabbergasted.)