I like the idea of increased diplomacy. It doesn't necessarily have to be through the UN. There needs to be enhanced diplomatics options, in general.
One aspect that I do see working with an enhanced UN wonder would be declaring war without taking a reputation hit. At the start of the modern era, whenever war is delcared you should take a reputation hit, unless you are defending an ally. If you get a simple majority of UN nations to approve then your declaration is approved. You get the UNs blessing.
If you fail your vote attempt, and still declare war, then you lose rep with those who voted against you. You keep your rep with those who supported you.
In diplomacy I would like it if there were an attitude spectrum like in Masters of Orion.
Also "jealousy" needs to be tweaked down in Civ 4. Powerful nations who oppose you should be jealous, but not every nation. It gets rediculous in some games when you've nothing wrong, but everybody is still annoyed or worse.