Residency requirements are entirely reasonable, and every US state used to have them for access to almost all welfare services, hunting and fishing licenses at the price for state residents, and etc. However, that alone might only delay the threat of complete unitary power over the witnesses until an attacker met the requirement. I think some other metric will be necessary to reflect the will of the Demos so that stake alone isn't able to seize unitary control of governance. Adding residency and activity requirements to witness voting can prevent the sudden seizure of governance by a well funded attacker, but wouldn't prevent the eventual seizure of power. As long as Hive remains a pure plutocracy all that is necessary to seize unitary control of governance is more money than 50% of the extant stake.
Some form of oracular verification a user/account is a unique human that votes a Hive stake must sort people and enable individuals to vote their preference - not only the weight of their wallet - is the only idea I can come up with that prevents nothing more than more than 50% of money voting for witnesses from taking - and keeping - totalitarian power over the platform.