I would like to see more U.N. options also.
Building & Being part of the U.N.
Say you reach the tech level to get the U.N., you can only build it if you have not been a cheat and an aggressor nation for say the last 40 turns. If you have been, then you must be a good civ for 40 turns or so to build it and hope no one else builds it first. After its built then anyone can apply to be part of the U.N., and if you have not been attacking or back stabbing everyone in the last 30 turns, your civ may get voted in if you have the majority of votes from current members and can be voted out the same way for bad action. The U.N. should be made of peace full and loyal civs that shun all aggressive warmonger civs.The civ to build the U.N. would be the head of it and have 2 votes at each session, if the city with the U.N. is captured then the U.N would still exist and be moved to the next city of the same civ until that civ is wiped out, then it would go to the next highest ranking member in the U.N.
U.N. Members if attacked
Once in the U.N. If you are Attacked you can call a summit and ask ALL members to come to your aid in many ways WITHOUT OBJECTION (all out war must be voted on). This may include breaking diplomatic relations ( No contact, like during a war ) with the aggressor civ ( if this course is taken, it is the last step that can be taken before all out war from all other members of the U.N. would be asked). Before asking the U.N. to break relations you may just ask them for an embargo against the aggressor civ first( includes right of passage ). Also the members of the U.N. that may have had trades or other deals with the aggressor civ, that have been broken due to these actions will NOT be marked as a cheat.
Aggression policies
If you ask the U.N. to go to war with the aggressor civ( one that YOU are all ready at war with and has attacked you first), there are a few ways to go about it, but one will happen.
1. Is all out war with ALL members from the U.N. for unconditional surrender of the aggressor civ( war to the death, aggressor civ must loose ALL cities, no peace un till this happens).
2. Is all out war with ALL members from the U.N. to cripple the aggressor civ to make future war, by taking all but say 3 or 4 of their cities, but not taking the capital. When this is accomplished then all U.N. members declare peace with aggressor civ.(no peace un till this happens)
3. Would have ALL U.N. members go into war economy and supply you with defesive units for the course of the war and they would be in the secret war(not declared on aggressor).
Whatever way you go there must be a vote from all U.N. members for them to enter the war. Depending on your situation will determine the outcome of the vote. If the aggressor civ did a sneak attack using your ROP or you lost two cities in the first turn of the war, the vote may favor wiping out the aggressor civ. When you are ATTACKED and call for a vote for war one of the 3 options WILL be picked, A vote can be called once per turn when you are attacked and at war with an aggressor civ.
When the #2 or #3 of the aggression policies is voted and passed weapons inspectors are often used in the post war to keep the aggressor civ from building an army any larger than one needed for self defense, or if the aggressor civ has used weapons of mass destruction in the past war. Inspections can be conducted on an unannounced basis, with strict sanctions or swift military action from the U.N. members if inspections find violations.
U.N. members wanting to attack
When not at war all action must be voted on, this goes from embargoes to your civ wanting to declare war. If the vote passes then ALL members will follow your cause just like if you had been attack. Even if a vote fails it can't stop you from making embargoes or alliances with fellow members that voted for it, but you and the other civs that go along with you will not get support from the other U.N. members that voted against it and get a bad rep or could be forced out of the U.N. by a vote, from past bad actions.
When 2 or more U.N. members go to war with an aggressor civ they declare war at the same time and declare peace at the same time. This goes for all parties involved.
Draw backs
Membership has some draw backs too, you can't make mutual protection packs at all, and no alliances or embargoes with non U.N. members. Also members are not allowed to use a spy. Hard to get in the U.N. but easy to get out, just withdraw your membership, you will suffer a bad rep with the current members and not be allowed back in for some time but can carry on your own agenda.
Non U.N. members
Non U.N. members may ask for support from the U.N. in the form of 1 & 2 under the Aggression Policies,and also embargoes, and breaking diplomatic relation. A summit will be called with all U.N. members and if the vote is majority then ALL U.N. members will join your cause, but if the vote is NOT the majority, then you are on your own, without any of the U.N. members joining you. If you have made 2 or more request, one may fail but one may pass(no war but embargoes). If action is taken then all of the parties involved will declare war and peace at the same time.