okay so i have had thoughts abt prsk bsd au many times. they've almost all been young skk SEKAI born from their shared and Strong but conflicting feelings of trying to find a reason to Want to live (dazai wants to die but chuuya, esp young, is still chasing a reason to want to live outside of duty honestly). oda is one of the people brought in by them soon after, probably. perhaps one of the flags, who i should tell u about i think, like albatross or pianoman... but its hard for that bc theres the disconnect between chuuya and dazai's friends. miku and co have their work cut out for them so hardcore. i think the miku would be like oda somehow, though. even though he comes later. hes just.. i think hed be a good friend to both of them if it was allowed at all. and that would be essential to have even when both of them have to be reflected somewhere too...

chuuya and dazai would get too many duets. they sound good together (really and genuinely) and also theyre skk so like.

and theyre all just.. normal, fucked up people without powers or crime and such. so oda probably survives but moves on as an adult a Bit older than skk and dazai cuts contact by necessity when he flees his <3 shit adoptive father at 18. yayy

and now for more of what u actually asked for.
first and foremost, in any ADA SEKAI, the miku is like a subdued version of kunikida which leads them all to be like. "ah, just like our president" bc both of them are hardworking and of similar thought processes, honestly, despite fukuzawa having internalized thoughts versus kunikida's very Outward thoughts that happen bc of how easily he can be flustered.

ranpo and yosano and kunikida with a SEKAI to start and dazai gets to join After... but ranpo with his sekai for just. years until he grabs yosano and lets her in then lets kunikida in then lets dazai in for the lols. dazai like wow i have two mikus and this one doesnt have a REALLY depressed len kagamine next to her.
baby dazai <-> len
adult dazai <-> kaito

ranpo <-> len
kunikida <-> miku
yosano <-> rin, but a little bit meiko, as she gets older.

chuuya <-> luka bc i want an unhinged luka i listen to enough rock vocaloid to want this.
oda <-> miku
i could think more abt the flags but i wont yet. yet.

the port mafia sekai is... almost the empty sekai, but its like a field. and as feelings grow, more and more flowers start blooming...
ada sekai is jsut a different street sekai. not the same area:) different street. a comfortable coffee shop...

AS FOR THEIR SEKAI ITEMS? hm.

dazai (baby): just. discarded bandages. maybe he has a moment where he loses his eye bandages while talking to miku.. they just stay.
chuuya: a pencil. just a pencil. there's a bit im thinking abt for that and im too tired to remember it super well but i will expand on this later perhaps.
oda: whole ass table with food on it. he brought it in to stay. feeding miku and the boys.

dazai (adult): this is harder honestly. it might just be a rolled bandage instead of an unraveled one. i think that has some... symbolic change even when hes covered in them still.
ranpo: all his important things are on his person!! so snacks get left in the sekai. for him. because all three people allowed in the sekai know that he's not afraid to bite their fingers if they steal his snacks.
kunikida: notebook pile notebook pile. some of them are notes on peoples. none of them have his ideals lists. not allowed to be seen AGAIN..
yosano: im torn between her butterfly hairpin on the table and like... wet specimen bc she canonically collects those but like. in a sekai.. hm