I like the large maps and find the constraints to the colonization rush too limiting. I like building my empire as fast as I can, but the game makes it difficult. Especially since the computer races also try to build up their empires as quickly as they can. Each colony has a monetary cost that has to be offset by taxes from colony's population, and a new colony depends on support from the empire's coffers until that crossover point is reached. Not to mention how much building buildings moves that crossover point. Right now, I think GC2 has it at a good balance.

This is one of my hopes for GC3, that diplomacy takes a step forward in terms of depth and mechanics.  Diplomacy in these Civ type games is always so basic and seems to be thrown in just because you need some way to communicate with the other Civs, but it seems so little effort is given to actually making Diplomacy an interesting, fun, strategic aspect of the gameplay.

Being able to barter over anomalies, asteroid belts and planets would be a great feature.  Offering promises of "spoils of war" would be a great addition to rally allies to your cause.  Forming temporary coalitions as opposed to permanent alliances would be a great addition as well.  Perhaps a coalition is formed just to defeat a particular race and then after that is accomplished the individual member races of the coalition go on their separate ways.

However, what I would really love  to see is that communication matters in diplomacy.  I'd love to see a "mini-game" made out of diplomacy.  Perhaps a screen where you are presented with a few different options in terms of how you communicate.  You could select different phrases, then select a type of demeanor.  This would mean that you really have to be careful about how you communicate with the various races.  Perhaps some races respect a more authoritative tone, while others may want you to show more subserviance. 

The whole Yes/No/Request aspect of diplomacy is frankly really boring.  I think by adding more elements to Diplomacy you can then actually utilize your level of diplomacy research in the game.  If you know more about about communication and diplomacy and even through spying know more about a particular race, new options for communication open up for you as you progress through the Diplomacy/Communications tech tree.  In the beginning you're left to kind of guess as to how to communicate with the races, how will you phrase your request or your statement and what demeanor will you use?  Then as you research Diplomacy and learn more about the species from spying the Diplomacy "mini-game" opens up more options for communicating, gives you more subtle/detailed options for communicating and gives you something like a "chance of success" type read out.  Your advisor could say something like "The Drengin are more likely to respond to a humbled request sir."

As you become more powerful as a economy, as a military and as a diplomatic body, new options for communicating open up and the way in which the various races respond to you could change.  Perhaps early in the game, when you are weak the Drengin will respond favorably to a humbled approach, but as you gain power and perhaps progress beyond them, then then respond more favorably to a stern approach.

These are just quick ideas off the top of my head, but I think that Diplomacy can be changed/improved to be a legitimate, strategic and enjoyable game mechanic and not just something thrown in to list as a feature on the back of the retail box.

It would certainly be nice to have a greater range of diplomatic options, including things like real trade embargoes, LIMITED time (and variable scope) economic and research treaties, and the aforementioned "chatter" communicating intentions.

About the only thing I can think of off the time of my head, however, which will slow down the immediate colonization rush is to make colonization radically more expensive.  The fact is, under GC2, after about the initial 25 turns (gaining Ion drive and some basic economics), I could throw out a colony ship from my homeworld about every 5 turns, 3 if I was lucky, and still keep my budget under control.  Colonization needs to be much more expensive, and it should take MUCH longer for a colony to be able to become productive.

Couple of ideas here:

1. Make the colony modules 5-10x more expensive, and only carry 1m people or so.
2. Have an early "transport" module available, one which is only useful for moving population around, not for invasions. Say able to carry 10m people per module, with space requirements leaving you able to put 1 Ion drive, 1-2 range extenders, and 1 transport module on an unimproved cargo hull.  You'd have to get a nice little shuttle service moving population around, with this kind of thing.
3. Make the initial colony module low-capability, and requiring improvements.
1. Maybe 4mp, no Research, 1 food.   An intermediary upgrade would make it 8mp/4 R/2 food.
2. Have a research tech required to learn how to upgrade initial colony buildings
4. Make the initial colony module PLACABLE on the map - the player should get to chose where to put it.

radical. Very radical.

yes agree this pissed me off no end in gc2

"you dont have any military ships you're physically weak if you dont give us credits were going to declare war on you"

umm yeah about that just gonna quick build some fleets on my planets and... oh look my military strength just jumped to triple yours now how about handing over those credits

I think this one is interesting, you basically declare a protectorate on another civ. Basically a lesser state of war, whereas I will only attack things in my neighbors influence that is at war with my neighbor.

I like the concept.

As far as colony expansion goes, i would say maybe a chance modifier.

eg. if colony is 5 parsecs away a 90% chance and 50 a 9% percent. Meaning you would have a better chance of colonizing home territories. I would also add something like if colonization force is greater than 5000 than 100% or something like that. Maybe have modifiers linked to relations too. Maybe cultural influence by treaties or friendship could effect as well.

as to warning the ai, i would love to be able to scare away scout ships. Perhaps something like civ 3 remove your ships from our territory or face diplomatic consequences.

I think that if your trying to pay someone to go to war with another race, their allies should be included in the talks, with possible negative/positive consequence same with peace treaties. I never found it easy to make peace in GC2, it was always pretty much til the death. 

Perhaps if you sold tech to a race, you would be given a certain leeway in future negotiations. It would be great if you could lobby for colonization of a world in someones territory.

I think we are both on the same page. It just comes down to: "If you attack my friend, i will DEFEND my friend. I will not attack the aggressor's territory".

A lesser state of war effectively.