Reference Imperium Galactica 2. I'd like the entire act of planetary invasion and tile takeover modeled from that game.

That's more of an idea for GC3.

I don't agree. dividing all of your planets in to two different populations adds way too much management at the planet screen. IMHO if anything is changed in planetary combat it should be at the tactical, and not the management, level.

I wouldn't want a blind rock-paper-scissors model like MoO3 had, it was mostly just random but eventually I found out that clicking 'Flank' or 'Ambush' were the only valid options. Sitting here thinking I thought the most compelling idea might be a way to 'purchase' greater advantage by spending money, and then I realized that system is already in place in GC2 in the form of MiniSoldiers, Tidal Disruption, etc. It might be interesting though if the defender also had tactics they could purchase? Just a thought. The defender could even have scorched-earth tactics like the attacker has, or maybe a 'go underground' option that diverts say 10% of your advantage away (doesn't matter if you were going to lose the planet either way), but gives you intel and influence on the planet after it's conquered.

Yep, I would LOVE to have the Defense equivalent of Invasion Tactics, but I wouldn't make it completely blind.

One side should know what the other is picking before they pick there own tactic in some cases (if they high espionage level, for instance)

You could Mod barracks as increasing your planetary defense, that functions like your population will be trained for war, in reality when a planet would be invaded, I think the population (the man and woman) who are physical trained (that means in a young age 18-45) would given a weapon to defense their planet.

but to divide planets population in military and civilians will be too much micromanagement in my opinion.

I hate to bring this up but how did MOO2 handle this aspect of the game? Its been so many years since I played it so I cannot remember anymore. Might be that my suggestion below is just the MOO2 system, dont remember really. Yes the system below would increase micromanagement slightly but in my mind it would make the game much more intresting, so bear with me

How about have those barrack buildings enable training of Legions? Legions would take much time to build (they could be in building queue) and cost much. You would just have one more text box in planetary screen telling you in example "Legions on planet: 10" or so.

When you attack civilian only planet with Legions there would be some minor casuality numbers on both sides and you would be presented with moral choice of "exterminate(evil), leave some(neutral), everybody lives(good)". Obviously all these decisions have their ups and downs. Exterminate and you lose all tax payers but have no fear of rebellion etc. Leave all and you swim in money but have leave Legions to guarantee the peace.

In the other hand if that planet had civilians and legions it would be much tougher. There would be much more civilian casualties and the battle would end when other sides Legions run out. After the battle you would get the same kinda moral decision to make, "how to handle rest of the planets population". The transporting system would need a bit change, maybe so that civilians could be transported only with Colony modules and Legions with Troop modules only?

I don´t want to bash the current system too much, but you must admit, slapping billions of citizens into transports with laser guns and send them to conquer alien planet hardly sounds reasonable. I would suggest that SD drops the time eating Epic Generator and focuses their attention to improving the actual gameplay. I think the time could be spent better with improving land combat, espionage, constructor handling, UI etc. This is the best I can come up with Land Combat, please add your own insight about suggested systems or just write your own system example

I do agree that in some ways simple is better. But to be honest I got bored of GC2 because too many things of the game are presented overly simplisticly. Diplomacy, espionage, land combat, space combat are all rather simple. The deepest part of the game was perhaps the ship building, which has runned out of steam, atleast for me. I do like the templates very much!

DA with its improved diplomacy, asteroid fields, espionage and terraforming techs sure are great improvement. But I really would like the gameplay get deeper too. Werdell did mention that he wants to create a game which you can play for years.

edit:

Have to correct that diplomacy is not that simple in GC2. Compared to other games (most reacently MTW2) I have no bad things to say GC2 diplomacy

In my opinion I feel that during an invasion the very last thing that would be factored would be the ground troops on either side. If said planet knew of the impending invasion, which it would/should, then there would be a decent selection of "planet to space" weapons in which to take out some of the invading fleet. This should not be left to ships in orbit alone.
Only after touchdown and deployment of invading forces would a standing military come into effect for defensive purposes. This would obviously be the second phase, or more of a second chance. One would assume that given the high technology present, there would be various methods of planetary defense to rely on besides actual troops since it is up to the invader to conquer using troops to hold his ground. This isn't the same for defenders, since they already possess the disputed territory (remember the Alamo anyone?), hence they wouldn't necessarily need superior numbers, just superior defenses.
Maybe if there was a way to look at invasions from this point of view, more ideas would come forth?
And forgive me if I'm rambling...  

Planet-to-space weapons would be a nice idea as a defensive technology. However, because even planets such as Earth are large enough that you would need a lot of ground-based cannons, there would have to be a limit to how effective these weapons can be. An assault fleet would likely stay out of range of ground weapons while it engaged the orbital defences. Therefore you'd only get a chance to do some real damage against the enemy as he attempts to land troops.

When the assault fleet sends the transports in, it will of course provide covering fire and attempt to knock out any ground-based defences that are detected. This may mean that it will be useful to keep the transports grouped with their escorts when commencing the ground invasion.

If a transport isn't covered well enough (ground cannons > space cannons) then it may crash or not land where it was supposed to land. This results in either a loss of attacking troops, or a loss of advantage.

However, because even planets such as Earth are large enough that you would need a lot of ground-based cannons, there would have to be a limit to how effective these weapons can be.

Even better, let's dust off the trebuchets and ballistas. And sticks, we need lots and lots of pointy sticks... sheesh! CANNONS?!? As you can tell this has me a little bit pissed...    

We have two AMS's built by lockheed right now to defend against all the nukes possible launched at the US...

I would think that an anti air-craft missle system would be just as feasible if not more supposing we're able to research all these other far out techs in the game.
Personally, I would just like to see the planetary defense tech tree grow.

I hate to bring this up but how did MOO2 handle this aspect of the game?

I haven't played it in a few months now, since despite not many games being able to replace it, either from being poorly concieved, pporly carried out, or buggy as Hell, but IIRC;

Each planet had up to five ground units slots, based I believe on max pop.
To begin, you have no ground forces (or just militia? I think large pop worlds got militia, can't recall) untill you build a Barracks, when you get infantry. Those infantry can be left to defend the planet or loaded onto transports to invade other worlds. The wepons and armor used by your ground troops correspond to the best ship's armor and best regular-sized ship weapon (ie, not plasma or ion cannons, or Black Hole Generators, etc.). Later, you could replace your infantry for tanks, powered armor or Titans, though truthfully, I've always found it much quicker and more efficient to load up on bombers with Bio-Weapons and scour enemy worlds. But only because the ground combat in that game was uninspiring to witness.

On a side note, many people seem to have concerns over the level of micromangement, in that too-complicated systems of ground combat would create more micromanagement.

I know MoO3 tried and failed with this, but I avoided it, so I'll pitch the idea:
What about Ministers would could take over the micromanagement of areas you didn't want to personally handle? You could give them all very explicit instructions (telling your General to mass and army of x units on SoAndSo World, to prepare for an invasion of That Other World, while distributing out forces to your other planets to ensure they all each have y units, aside from potential targets, which need to be beefed up to z units)

Perhaps we should add "opportunity to crush rebellion" on the list of things to add.

YES! I would love to slaughter those tree huggers!