I agree that the UP style mechanic needs an update.
I absolutely hate how UP/UN/etc mechanics are done in the vast majority of games. They are completely unrealistic and I feel detract from the feel of running an empire when some 3rd party can just dictate how you run your empire with no recourse. If someone makes a demand through normal diplomatic channels you can just ignore them with possibility of war to put them in their place.
I mean look at the real world, just because the UN passes some resolution doesn't mean all the countries have to follow it. There are often political tensions and other issues raised depending on what it is. But the point is the empires should have the option to not follow it.
The biggest reason I am against the forced following of passed "laws" is that it is basically a "win more" mechanic. As voting in these types of systems are often tallied based on size/population/etc which means the most powerful empire often gets whatever it wants which is something to benefit it's own interest.
The other thing is the whole idea of the UN is a modern concept where the countries all agree to basically put pressure on or attack a country who behaves aggressively in an attempt to prevent another world war from happening. In a game where war is common place and empires are at each others throats the existence of such an organization makes no sense. A neutral meeting ground for diplomatic talks sure, but not a body that passes laws which it has no means of actually enforcing other then the word of it's members.
If anything the whole mechanic should be more about alliance building. Things like empires getting together to form trade embargoes against curtain groups. But not just a simply vote and no one trades with that empire but more shades of gray.
For example empire A wants trade embargo not only on empire B but also any empire that trades with B, since politically open trade has always been viewed as a type of alliance because it gives aid to the enemy. Thus an empire can choose to join in the embargo against B and it's partners, or join in but not embargo the trading partners of B, or simply ignore the embargo completely.
In the example above each option would have varying degrees of political ramifications. For example let's say B trades with C. If you go with embargo all then A likes it but B and C don't. If you go with Embargo just B then A and C slightly likes it while B doesn't. If you ignore it then A dislikes it while B and C are happy with you.
Instead of the vote for/against something and winner imposes rules on all, I think a system that is more dynamic and player driven would be better. First off the random law comes up for a vote and players vote has to go. Your basically just at the whim of the random number generator on what comes up for a vote. However a system like Civ 5 recently added I don't think does much good either as in that one the top 2 empires get to pick what is being voted on and that is basically another form of "win more" since highest votes gets to pick but that also means they are most likely to get their votes passed.
I think two types of votes would work well in such a system. First in ones that the player proposes, such as the set an embargo against target player. In order to prevent players from spamming these types of voting calls some system could be set in place as to how often they can be called, perhaps the spending of influence points or such. The other could be the typical galaxy wide "laws" which instead of being entirely random have a list of perhaps 4-5 key issues and then each player picks their top 2. Then which ever issue got the most picks is the one brought before the UP to be voted on.
This way the players have more say in what the UP does as well as having more political impact on the game.