>DC doesn't want to be part of Maryland (they like being independent and feel that Maryland's concerns are not theirs) and Maryland doesn't want DC (they feel it would move the centre of gravity of the state even further towards the DC suburbs, making it worse for the further-flung parts of the state).
I think that's worth considering. It would get rid of a lot of the left/right gridlock at state levels. Cities would be mostly free to do as they wish without rural areas holding them back and rural areas would be free to do as they wish without cities holding them back. This would also greatly reduce the amount of BS that needs to get done at the federal level because a ton of that stuff is not interstate issues but an end run around the fact that the blue team and the red team can't do what they want at the state level so they try and get the feds to do it to the whole country.
And before anyone says "but money" I would like to remind them that freedom isn't free.
Bremen and Hamburg both were Free Cities in the Holy Roman Empire. More importantly, they were among the very few Free Cities that retained this status until and past the end of the HRE (1806). Being in the Hanseatic League helped establish their importance, but the League wasn't actually a major concern at that time anymore, and hadn't been for over a century, but they certainly played to that history (and e.g. added "Hansestadt" to their names, long after)
Every metro with population above median or average of existing US states should have the option of forming a new state. Seems to work for Berlin/Bremen/Hamburg in Germany, and various metros in other countries.