I'm in the process of reading China Miéville's fascinating novel "The City & The City", which is also adapted to a TV series. The plot takes place in a fictional twin city somewhere in South-Eastern Europe. The two cities Besźel and Ul Qoma have grown into each other in a weird and complex way, but they never really merge, and the residents have learned to see only their own city and instinctively ignore e.g. bypassers who are officially in the other one.
I can't help thinking that an OGF version of the idea would be exiting. Obviously, the original idea as such is both unmappable (how to map a tree which is partly in one city and partly in another?) and too sci-fi from versimilitude point of view. But some kind of complex mixture of two cities – perhaps with two overlapping but unconnected road networks – could perhaps be cool to create as a community project. After all, there are weird things in the real world, too – it would be more or less a combination of Baarle-Nassau/Baarle-Hertog and Cold War Berlin.
This is just a wild idea I wanted to share in case anyone takes interest!
(Side note: It is interesting that the name of one of the two cities "Ul Qoma" seems to mirror the Finnish word "ulkomaa" = "the abroad", but I can't find any verification that it is intentional. I thought at first that the word was an invention of the Finnish translator, and I was astonished to see that the same name is used in the original English version of the book.)