What would it take for some future US Government to cede over more American soil, parts of an established city even, to a new faction unanswerable to the Federal Government?
You mean like what happened in Riverton, Wyoming?
I imagine that sort of thing is going on all the time.
There's also genealogical testing with a moving goal post where some Indians are told that they are not Indian enough because of contamination to their blood lines (extraracialbreeding) that forces eviction from the reservation, tribe, and profit sharing with any tribe related enterprises.
But by "New faction" I meant "not Native American".
Random nationality generator... American Armenians want Land in return for some slight heretoforeafter to be called New Armenia and is unanswerable to the US Federal Government with out protracted diplomatic pissing and bitching.
Although the Norwegians might actually be able to retroactively call dibs if they got their shit together, and get some sort of settlement since they had 600 years on Christopher Columbus.