This has probably already been done before, but: Fanfic Prompt - Mandalorian Noir AU.
Mandalore and the Jedi would have to be taken from the source material and adapted since their cultures are too specific to be easily translated to a single real-world place, but you can make it work for everything/everyone else.
Setting: 1946-1950, or maybe later, but not by much.
Mandalore would be a small but mighty island nation that was invaded during WWI in order to be used as a navy and air force stopover and refuel base during WWI, and most of the surviving Mandalorians fled their destroyed home and developed diasporic communities.
The Jedi are a religious organization with a knighthood that operates like a nonprofit organization. They had their own sovereign base within a country, like The Vatican, but there were/are other smaller temples around the world, again like Catholicism. Concentrated efforts were made to wipe out the Jedi at the end of WWI.
DIN DJARIN is a noir private detective and bounty hunter. He did not fight in WWII, but he did help people escape both German invasion and the Japanese internment camps. Now that WWII has ended, he continues to help the oppressed and has been hunting down Nazis (Empire) and sympathizers in North, Central, and South America.
As a licensed foster parent and a Mandalorian, when Din finds GROGU, a curious young boy who can't speak, Din takes on the responsibility of fostering Grogu until he finds the young refugee's family.
LUKE SKYWALKER was raised an Irish American in an Irish American immigrant family (this is very important due to how the Irish immigrants, in particular, were treated, comparable to Tatooine under the Hutts and Anakin's background). After becoming a war hero, he is now working on helping his sister, LEIA ORGANA, in her efforts to form and develop the UN, and reclaiming the Jedi's version of The Vatican.
Like Luke, BO-KATAN KRYZE is also attempting to reclaim her home country of Mandalore, but she must unite the various diasporic communities of Mandalorians in order to kick out the renegade Nazis who are currently occupying it, since the Americans and the British claim to be too busy to assist. She recruits Din to help her.
Din spends the fic hunting down and killing Nazis/Empire, helping others, and teaching Grogu about being a Mandalorian, even though he is well aware that he is a foster parent, and the best-case scenario for Grogu is to be reunited with his family. Eventually, Din does find Luke, who agrees to adopt Grogu and train him to be a Jedi. However, the people who Grogu was originally rescued from won't let the story ends so easily, and to save Grogu, Din not only has to go kick more Nazi/Empire butt, but he also inadvertently helps Bo-Katan reclaim Mandalore and become Queen, and helps Luke reclaim the Jedi Temple. Grogu chooses for himself to remain with Din, wanting to be adopted instead of fostered. I'm not big on the ship, but Dinluke shippers could throw in some kind of joint custody agreement.
Other random bits and bobs:
Alderaan is somewhere in the Mediterranean, which is part of how Leia was involved in the war way before Luke was.
Bespin/Cloud City is somewhere in North Africa, kinda Casablanca-like. A minor storyline could also involve Lando Calrissian reclaiming Cloud City.
Leia and Han can be dating, engaged, or married in this fic. They appear, at minimum, when Luke, after befriending Din and hearing about Bo-Katan and Mandalore, gets Leia to help Bo-Katan get her diplomacy organized and get a seat at the table in the UN. Leia and Bo-Katan become good friends. Han is low-key terrified of Bo-Katan.
There's room for Dinluke here, but my personal headcanon is Asexual Din Djarin, so I think this is just as fun in the form of "Din Djarin Accidentally Makes Several Famous, Powerful Friends Despite Noir Monologuing About How Tough It Is To Be On Your Own".
