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

mizuki + rui. do i have to tell you about them? i love them so much its one of my favorite relationships in the game and i hope one of the first cross-unit friendships to get a title for. they were so important for each other and just helping the other last through their loneliest time.
mizuki + akito. its jsutfunny. their dynamic is so hilarious it kills me. my god.
mizuki + airi. they!!! are friends.
mizuki + emu + saki. imagine the POWER so much cuteness in one place could hold.
mizuki + tsukasa. im so glad they have an event with lims together now. esp after rui made the tsukasa noises toys to make mizuki laugh. i want them to be friends im looking at mizuki like snucklet until they say tsukasa-kun.
tsukasa + minori. fanfesta above all other events for really CUTE interactions... i really loved this one theyre precious and can cheer each other on please.
tsukasa + shizuku. omg childhood friends!! they could be besties now! they werent super super close as kids but tsukasa fuckign screamed when he saw her again. i want them to be BEST friends.
rui + an. i just want to see it. i just want an to hang out with rui i think they were so fun in the wedding event and she treats him like a normal person at school and i like it so much.
rui + saki. tsukasa fanclub. :)
rui + toya. the both of them really get through to each other in such a good way in the wedding event and i want to see more of them... i just want to see them really become Friends.
toya + nene. two quite people in two very different ways. two gamers. same class. they are so just casually friends that its perfect and they mean the world to me.
nene + minori + kohane + ichika. again fanfesta did something to my brain. theyre all so? well fitted to each other and in different parts of music but they learn so much from each other and each others friends that i want them to always be friends. send each other updates and track each others careers through adulthood. surprise people when they learn theyre good friends who go out for drinks (alcohol or coffee idc theyre in their adult years when they have careers) and talk abt things.
kanade + honami. they are friends by accident. taking a job to clean someone's house bc they cant handle it themself as a teenager living alone and becoming close since youre also a teenager is just... idk i think honami as a platonic or romantic homemaker who just helps kanade feel safer is really good and honami deserves someone who cares as much as kanade really does. nothing bad abt l/n bc they also care so much but yknow. more!
homani + emu. APPLE PIE KING AND QUEEN.
airi + saki. saki was airi's fan when she was in the hospital and them hanging out!! thats so cool! its even better knowing that saki and tsukasa actively knew shizuku, who is sooo close to airi, so they could just. all hang out... makes me happy.
im sure i missed MANY. bc i care about all of them:) but

• aquarium or zoo, butterfly garden too, if tsukasa can be brave for butterflies. watch as rui knows a lot abt biology.
• musical or play, naturally, but they would probably invite nene and emu and it would be less a date and more just hanging out while they hold hands.
• planetarium for tsukasa to excitedly whisper to rui about the pegasus in the stars and for rui to tell him he'll shine brighter than even that one.
• one of the restaurants that rui is a Regular in so they can see this really picky kid who used to come in So alone and sad with someone who makes his eyes So bright.
• somewhere tsukasa's worked as an extra before for the same, even if tsukasa's isn't as obvious. a bookstore for them to pick out books that look interesting to read for inspiration for their own writing.
• just a walk through the park either as its own date or on the way to or from any of the other dates...
• a picnic in which rui is brave and protects tsukasa from all the bugs (mostly, minor moments of exposure around someone entirely unphased should be good when theyre not bugs being thrown at tsukasa, right? ^^)
• i can think of more but this is getting long