I'd prefer more an UN that would work like a political arena where civs could push for their own advantages, backstab each other, grab for more power, and generally make the life of other civs more difficult. I mean what's the point of having a goody-two-shoes organization in a game of World Domination?
My vision of an advanced UN in Civ:
Once built civs get automatic membership (just to avoid some exploitation tricks...) with each having a number of "votes" or better called "influence points" based on the relative strength of the civs (number of cities, military might, population, etc). The builder of UN gets some bonus points to make building it worthwhile. Democracy is for the masses while international politics is Influence and Power so some civs will be more equal than the others.
In the UN any civ can propose an Edict. An edict can be:
- Ban building a unit/improvement
- Ban unit/improvement totally (=both owning and building)
- Ban the trade of a luxury/resource
- Ban a luxury/resource (=both trading and accessing)
- Ban a treaty form (MPP, MA, etc.)
- Declare war against civ X
- Give development aid to civ X (=everyone pays the proposed amount of gold or gpt to civ X)
- Other?
Once an edict is proposed another civ must second it in the following X turns. If it gets a second then the civs vote on it. If the "yea" gets more vote points than the "nay" then the edict takes effect and the civs have 3 turns to comply to it (selling buildings, disbanding units, initiating payments, declaring war, etc.) If they fail to do that they suffer an attitude hit and are shunned by the righteous civs complying to the UN edicts.
For example, if a civ didn't have access to ivory itself it might propose an edict to ban ivory (the poor elephants and all that...

) to harm its rival that did have access to ivory. If the edict passed then all civs using ivory must disconnect from them in 3 turns or suffer a major attitude hit.
An edict lasts 20 turns. It can be proposed again, of course. Civs you are in more friendly terms with will generally vote for you if it doesn't harm them while your arch enemies probably vote against you just for the principle of it.
(Well what if I _have_ played too much MOO3 lately?

The Orion Senate is a darned good game implementation...)