What would I pay good money for? Hmmm...how about the addition of the one thing that I was most dissapointed that GC2 didn't contain. Artificial Planets. Don't get me wrong, the new Tech Tree is nice and all, but there was a lot of stuff from the old GC1 Tech Tree that I was really hoping would be carried over and expanded, that wasn't. Artificial Satelites/Planets were the two I most wanted to see carried over and given a reall mechanical effect. Personally, I'd like to see you be able to "attach" Constructors to stars (that have fewer than five planets orbiting them) and begin construction of an artificial planet (maybe have the technology come off of Massive Scale Construction or...the last industrial technology, I can't remember the name of it). 24 more constructors and 25,000 bc (or so) later, you'd have yourself a shiny new planet to colonize. You'd probably want to make every artificial planet be a solid, un-terraformable class 10 for balance and common sense reasons. There are several problems with this, of course, and probably many of them are the reason something like this was never done, but...hey, that's what I want. Okay, I realize this is horribly wishful thinking, and not really in the spirit of the rest of the comments thus far, but a man's got to say what a man's got to say.
More realistically...Ai improvements would be nice (I'd prefer the AI played more organically), but are really not that big of a deal to me. My real request would be vastly improved and expanded diplomacy options. Much of that being that which others have already mentioned. The ability to threaten, politely or otherwise, other civilizations. Better trading (both with freighters and on the diplomacy screen). More peace-time options, better embargo-type options, more dynamic alliances. More relationship types (more than just War, Peace, and Alliance) like, I don't know, "favourite trading partner," or maybe just have the various stages of respect (Hostile, Nuetral, Freindly, Close, etc..) have more mechanical bonuses, rather than just being arbitrary names (that, admittedly, affect diplomatic trading to some extent). The United Planets is great and all...but it doesn't do anything. It would be great if it met more often/could be summoned by any civilization (player or otherwise) for a specific set of reasons (like, perhaps, new galactic emergencies), if it had more options for affecting civilizations (galactic embargos, war summons, yadda yadda), and if you could vote on multiple things per session (not just setting up a zoo and then going a way for a year).
Automatic civ-specific trade goods. (for example, the Terrans automatically have three things that no other civilization has or can research, let's say Cheese, Cockroaches, and Ceramics.) Each civilization has their own, unique Civilization Trade Goods that provide unique bonuses they can trade as they see fit. Just, generally, anything to further differentiate each race from another. Looking different than the other guy just isn't good enough anmore. Maybe cultural baises beyond what there already is. For example, maybe the Terrans get a discount (say, 15%) on researching Mass Driver-based weapons technologies and Influence technologies, while the Drengin get a similar discount bonus on energy weapon techs and invasion techs. Altarians get a discount on Diplomacy techs and Ethics techs...you get the picture. Maybe unique civ-specific technologies, like Advanced Holographics (don't ask me what bonuses that would grant) or something that only the Terrans (or whoever) can research...but only one or two per civilization so as not to get out of control.
Many, many more random events and (most importantly) them not completely shutting off the second you choose an ethical alignment (I hate that!).
There's more...but I think I've given enough unusable suggestions for one day.