Unit support should cost money, as in real life, and thus would be supported on a civilisation wide basis. It would also make massive military campaigns harder to run without any basic infrastructure - no bad thing. Finally, the desperate search for cash to pay soliders was one of the cool things about medieval europe - it would be amusing to include it in Civ as well.
Unpaid units would then either disband OR mutiny, turning into barbarians, depending upon the stability/type of government or the police rating or whatever.
The CtP type units should NOT be included. They present two problems with a civ type game:
1) Unrealism: The bad kind of unrealism, where suspension of disbelief gets stretched too far. CivIII will always have a good deal that is not 100% true to nature, but Lawyers and televangelists go too far. They don't exist as a political tool in the real world.
2) Countermeasures: CTP required too much work on covering for every possible kind of attack on every possible city - it became unworkable. Special ability units should be very limited in scope - the bulk of them should focus on assisting units and home cities. Only spys/probe teams are a realistic choice for offensive special abilities.