I'm currently setting up a campaign about a very Kowloon walled-city inspired cityscape that people are forced to go to to live out their days in exile from their home dimensions(?) as a sort of dumping ground for criminals and undesirables. Because you have to go through a dimensional portal(?) to get there, there is very strict customs and gun control meaning most people are just unarmed in this city full of foreigners and cutthroat criminals; and you also can't leave. So people began to turn to the ancient traditions of martial arts to survive and defend themselves. Basically it's a kung fu crime setting/Yakuza with some Fist of the North Star-tier overpowered martial arts mysticism in there for good measure.

However my problem is trying to figure out exactly where these refugees come from. I don't want it to be a prison planet with sci-fi (though that is certainly an option) because I don't want it to be "too" cyberpunk feeling and have that near-modern feel. I also really like the idea of having a small selection of nonhuman/strange character choices but they have to be bounded to be within the sphere of human ability and size (because of martial arts).

So I'm currently trying to figure out what the best set up for this campaign setting is. My current ideas are;
>Alternate timelines where people developed different martial arts cultures; mingle in the "everyone died of a plague so the city is intact but nothing else is left" timeline to dump undesirables
>Alternate Dimension setup to explain different laws of physics/"dimensional nexus" to allow for silly martial arts and maybe different human evolutionary paths
>Near-Future Sci-Fi with biohacking/biomorphing twist to explain certain elements
>Just leave it unexplained(?)
>Other?
Any ideas for this?