Both SS and Medicare should be state programs. The Fed should put together certain requirements that each state must meet (minimum level of benefits, for example). But each state can choose how best to design their program for their citizens. For instance, the design of a Florida program (with a high level of retirees) should look far different than that in Colorado.
It's what we do with Medicaid, where the Fed sets guidelines, but allows each state to tailor the program to fit. And, Medicaid is so successful that they chose to expand this program by almost 100% instead of expanding Medicare. Sure, they could have said, "Make Medicare available to those without insurance" and it'd have been 100% within their power. But, they said, "Let's let the states decide, since they seem to be saving far more money with Medicaid, while simultaneously serving more people."