1. Retirement plans such as Soc Sec
2. Medical plans such as Medicare and Medicaid
3. Welfare plans
4. National Guard turned into 50 state militias
5. Most road/bridge/tunnel building. If we're not going to continue to build out the Interstate Highway system as Eisenhower planned it, and all it is is about expanding lanes and keeping them in repair, then all of this should be kept at the state level
6. Anything related to education
7. Anything related to regulating businesses. If groups of states want to work together to streamline laws to make it easier for corporations to work across their borders, then great, but if they don't want to then so be it.
Probably everything done by the national government except the military, the federal courts, the federal prisons (but we can definitely defederalize a number of crimes and return those prisoners back to the states), NASA, NHS, and other long term research support for the good of the nation.
And this fulfills the Constitution’s requirement that each state is guaranteed a republican form of government how? Article IV Section 4 does not say “a socialist form of government”.