Not sure if this was mentioned, But I think there should be a limit to the number of these Units you can build.
If you are going to limit their construction to 1 Civ. Your giving that Civ an incredible advantage. Essentially you can move These Units onto Key Resources of an Rival who you are at peace with, Declare War, and the Rival is cut off from resources. Then invade with Normal Units.
Thats what I would do anyway. Not use them as "Peacekeepers", Use them as a hybrid Spy/Special Forces unit that can effectivly give the attacker a HUGE advantage in a conflict.
Also for what its worth. I believe that if the unit appears as a barbarian (Hidden Nationality), then they can be attacked (by Ally's, or Rivals), the AI will think its a Barbarian, regardless if you are at peace with them or not. (Thats the point of Hidden Nationality). IMO, they should not have Hidden Nationality. UN Peacekeepers always have a flag patch on their arm of their home nation. I assume you can set them to "explore rival territory", like Explorers/Caravels, but they wouldn't be able to "block".
I like the Idea, I just think it would be hard to implement. I thought for a while that the UN can be represented as a Corporation. (Having the UN Headquarters Found It), and have it spread via a "Diplomat" But again, As the founding Nation why would you want to give another nation the ability to produce a unit that IMO is overpowered.
I thought about this, but it's all a matter of strategy. Obviously, building a bunch of UN units will force you to NOT build regular units. That still leaves you with a huge disadvantage if you are attacked with overwhelming force. In testing, I found that UN units aren't really the best units to have--hence, the reason why are cheap to build. Because they cannot attack, they are significantly worse than regular units of the same type that CAN attack.
I believe they are balanced as is, though I am considering the removal of "Hidden Nationality". I will run further tests on the theory.
As for your comment on making the UN a corporation... I was thinking the same thing, but then I realized that they really don't function quite like that. The last thing I want is for the UN to show up in your corporation screen. I don't know enough SDK to implement a special design to add windows and such. What I will be doing is the following:
1. The United Nations builds Diplomat
2. Diplomat builds UN Mission
3. UN Mission builds UN Peacekeeping Forces
I will be recoloring the Corporate HQ to represent the UN Missions. I will attempt to limit them to capital cities--though this may take some skill beyond what I have.
As for your comment on WHY you might want to spread the UN around... I will be attempting to implement a culture bonus for every UN Mission founded that is added to the city that holds The United Nations wonder. Also, I would eventually like to see a possibly permanent increase in attitude from that Civ towards you. Besides that, maybe you've vassalized a nation and you want them to assist you in making these little guys.
If anyone would like to help me make a working mod of this on the SDK, and have knowledge of how to code, please contact me ASAP so we can work this out! Anyone is welcome to participate in this project. Being that these units drastically affect the way the late game is played, I'm sure that many will be interested in downloading. Your name will be most definitely credited!
Thanks for the interest.
UPDATE:
I've basically been able to do everything I just talked about further up in this post except for one thing: Allow the UN Mission's construction in another Civ's city! Right now, you can establish a UN Mission in your own capital, but that's obviously not the point. If someone knows how to add code allowing you construct buildings in rival cities, please let me know so I can get your help on how to do this!