I think the future UN could stand to be a massive expansion upon the current in-game idea.
A much broader range of possibilities of things to vote upon. More civics, more events, things like that. Take a look at Galactiv Civilizations if you can, that has a number of interesting ideas in their similar organization.
Perhaps a unit or two only available to the UN.
An it would definately help to change the diplomatic options of the game as a whole, which could definately help with a UN kind of thing. For a simple example; a country which rules a powerful UN would be more able to cajole things from other nations, as said nations would be fearful of the popular opinion swinging against them if they refuse. On the other hand, a weak UN would generate slightly more antipathy towards the civ who makes demands, as they would be seen as sanctimonious at best and exploitative of their 'moral right' at worst.
As to other matters, what about the possibility of creating maps in a manner which creates solar systems? I expect this would be hard to code, but let me explain what I think could be done.
First, it would be necessary to create a large number of new terrain types. 'Solar' would be a star, and would be uncolonizable (Until very late-game techs are researched, allowing high-atmosphere colonies to be built.). There could be a handful of different terrain types for planets, for example one could be 'Fertile' which would create a planet similar to Earth, largely plains, grassland, and rolling hills. And so forth.
Then, code it so that a star is created, and a random number of planets (between 2 and 6) appear, not too close to the star or each other, around it. This would sadly require a fairly large map, especially if the planets take the form of 3x3s. Nevertheless, this would allow for a couple of civs to start in a snigle solar system, allowing fairly early contact, whilst ther civs would be further away and contact would be delayed until later, when better space travel tech was developed.
This would also require the development of a number of new tilse for space itself. Close-range space tiles, those between planets, would be more easily traversed than those between solar systems, hence the need for better technology before distant civs are met.
An alternative idea would be to create a map generator where there were two seperate worlds, and with space in between. This would work best if the plan is to create a mod which is an extension of the main game, rather than a stand-alone mod consisting only of space mods. There is the problem of being directly adjacent to a space tile, however... this would be ideally solved if it is indeed possible to create a series of overlaid maps, as queried in a thread somewhere around here... sadly I can't track it down right now. =(
Actually, this has me running up all sorts of ideas for a mod which takes Civ from the typical start all the way through to the distant future...