UN inspectors would be allowed into any member country, to inspect potential human rights violations, nuclear and chemical weapons development, and large environmental hazards. They need to have teeth to enforce things.
The Peacekeepers should be a decentralized organization with only three main rules:
1. Your only job is to defuse existing conflicts and violence, and force warring sides to resort to using courts and diplomacy.
2. Anyone in the organization found breaking the first rule (using force to take sides or escalate a conflict, like Norriega) will be put on trial and executed if found guilty.
3. Any alumnus of the organization breaking the first rule is likewise hunted down, captured and put on trial. The only difference is, if they are not on active duty, they aren’t even empowered to carry out Rule #1 fully but must call on-duty reinforcements.
Don’t like it - don’t join the peacekeepers. Every cadet would be trained in these three rules, so no one can take over the peacekeepers and rule the world.
And there is a set of rules that all countries can abide by:
4. If there is a sectarian conflict between groups A, B and C in a region, only peacekeepers from countries and backgrounds with no serious stake in who wins can intervene. Others are sanctioned / excluded from that conflict etc.
5. The peacekeepers wear a specific and unmistakable white-themed uniform. Anyone in any country caught impersonating their uniform would constitute a criminal offense, like impersonating a police officer.
6. They all wear body cameras, they present themselves and warn any armed combatants or terrorists to put down their weapons or they will shoot to kill. They are also able to pre-emtively kill in certain situations where they can prove massive harm (eg a bomb) and that a warning could have allowed it to happen.
This monopoly on force would probably lead to a net reduction in harm, assuming it can’t be taken over or corrupted.