Most services of government can be replaced and run more efficiently with the proposed decentralized models. There are some functions, however, that benefit from some level of centralization and thus a governing body via consortium is required.
The Regional Regulatory Consortium (RRC)
The RRC is a governing body that would manage regulation of commerce by region. Examples of current government functions which could be replaced by RRCs include regulatory agencies (e.g. FDA, SEC, EPA). The mandate of a RRC will be to ensure that commerce is conducted ethically and without abuse from bad actors. This includes consumer safety, environment sustainability, licensing, and other functions. It should be noted that many of these regulatory functions can be funded by charging industry user fees, and thus funding from a centralized general tax pool is not crucial.
Defense
Even in a target state model where nation-states cease to exist, there will still be bad actors that organize armed groups of people attempting to amass wealth and power. Thus, a defense force is required to maintain order. A professional military which could represent people across a region or the globe would be established to thwart armed aggression. Each Administrative Region would help fund the defense consortium either through a fee collected from its citizens or a group of reserve troops ready to serve in the event of an armed aggressor.
FDA at a Glance|https://www.fda.gov/about-fda/fda-basics/fact-sheet-fda-glance