Okay, here are a few ideas:

1)- Population growth should be max to an average of +5%/year. Making the pop. growth very very slow. That way, homeworlds are always top-priority worlds, and colonies will always be limited on the population, but have a great power military/research (think of Asimov's "Robots and the Empire", Earth's colonies posseded about 95% of Earth's military power, while the Earth was the majority of the population). The Homeworlds are the Heart of your Empire, while colonies are the arms.

2)- If there was more minor races, and the upper idea was applied, it would change a big dynamic of the games. Minor races would be precious tokens the Major Races would fight over, because they possess all the power of a homeworld.

3)- In GalCiv2, right now, trade is used to make friend. I am on the side that thinks that it works the other way around, you make friends in order to trade. If you have a high population/stability (for stability, see point #12), you should have more available trade routes. The UFP still limits "extra-territorial" # of trade routes (between empires), but within an empire (a.k.a. the Homeworld, colonies and Minor Allies homeworlds), there should be only the capability limit.

4)- Here are hypotetical trade treaty between 2 nations:
- Blocus (not a treaty, but still applies)
- Restrictions (1-2 trade route max)
- Favored (5-6)
- Free Trade (unlimited)

Star Trek: Birth of the Federation used much the same kind of dynamic. Colonies were really weak at the beggining of the game, but they were long-term investments. The best power (at beginning) were those who took control of the Minor Races planets, of stroke deals with them, since they possess high-population planets.

5)- On the topic of "allies", there should be many levels of alliances. Military being the one we all knows, a defensive mutual-protection pact. But there could also be "technological allies". Each ally put some technologies on the table that it's ally can use at will, but doesn't OWN it. Example: I exchange Phasor V and Tri-Strontium to the Altarians, while they trade me Industrial Sector and Barrier II. From now on, I can use the new technologies, but I cannot research subsequent ones (unless I climb the tech tree myself). Each ally can withdraw from the treaty at everymoment, thu taking back and giving back each tech.

5b) A civ owning a Trade Good should only RENT them

6)- Invasion forces should be much smaller. A small invasion ship should be able to carry 5 million population, max. At the end, the best invasion ships could carry up to 20 million population. (when you think of it, it's still incredably huge). You have to draft population in order to make them go into invasion forces, thu lowering morale.

7)- There are 2 ways of getting a regular army. Draft and Volountaring. You have to build a structure called "military base" for volountaring. You put a maximum quota of acceptable troops if you don'T want to bankrupt yourself with zillions of troops. the # of millions of people volountaring depends on a lot of things (civ's courage, if at war or peace, approval rating, etc...). On the other hand, you can Draft population at almost any time, any planet. But you are stuck with the draftees for a minimum period of time, and have to pay them. In addition, you loose morale on the planet.

8)- Any regular army based on it's native world also switch sides if cultural conquest is achieved. If a foreign army is present on a planet who tries to switch sides, it is now considered occupation army, who artificially maintains the planet on your side.

9)- To defend against those invasion, you HAVE to draft your population. Transforming them into regular army, who cost you 1 BC/10 million population/week, tax-free. The "draft contract" could last about 6 months. (Ennough to put a drain on your ressources, but not ennough to bankrupt you).

10)- Invasions are not resolved in 1 turn. Every turn, attacking and defending armies clashes, and the Transport ships have to stay into orbit until the military campaign ends. The defending army has the option of drafting even more soldier, but it lowers the planet's morale. The attacking army can use the support of orbiting ships for planetary bombardement, increasing it's advantage factor.

11)- Every turn, the defending planet has a % of chance of surrendering to the attacking force. The % is based on the planet's morale, and the money put on either side to make the population to either stay loyal, or to switch sides.

12)- A surrendered planet is artificially maintained into a loyal state. Which means it requires an occupation army. The planet has 2 morale stats. The "Stability" morale, and the "True" morale. The stability morale depends on the # troops stationned. However, the more troops you station on a world, the more the "true" morale lowers. If the "stability" morale is kept high for a long ennough period of time, and the native-civilian population begins to increase, the "true" morale will eventually rise up.

14)- If the "true" morale is low ennough, resistance soldier will try to harass your occupation force. You will lose X million soldier every turn, lowering your "stability" morale.

15)- If the "stability" morale breaks down sufficiently low, the population begins to take arm against you, and armies begin to crops. The more there is fighting between occupation troops, the more the "true" morale lowers. It's a hate spiral..

Edit: to change the title to a more appealing