Invasions are definitely one of the weakest mechanics as is.  These suggestions are good.

No, its all to complicated.

I like the idea including the possibility of joint ownership and stalemates, but I wouldn't wait more than 5 turns to conquer a planet. The devs have been mostly quite about PI rules so maybe this is already in there master plan?

Edladner: If you have fought through the defending fleet and now have supremacy of the skies, the defenders could not last overly long. 

Iraq, Afghanistan, Vietnam, etc. show that superior force of arms and tactical position is inadequate to predict the length or success of a military mission. Regardless, it's a game with aliens about space warfare. What makes sense is kinda secondary. I'm thinking about what makes for interesting strategic and tactical gameplay.

You have at the minimum 2.5 billion soldiers (in gc2 I often used 10 or more transports full of troops, the troops not killed moved on to the next planet) attacking planets with the usual avg of 15 billion inhabitants.   In GC3 I have 3 sizes of transport 5, 10 and 20.  I move in with fleets using what I need to overwhelm each planet.  I also have battlefleets running interference and all my transports are armed.

It sounds as though you've already won the game at that point. I don't see this mechanic having any noticeable effect on this strategy. Your masses of troops would quickly overwhelm the planet's resistance rendering any rescue or reinforcement operation moot. Not that they could do anything since your fleets would destroy them before they could approach the planet.

This is more about a situation where you are roughly at parity with your opponent. (Or the silly problem of planet sniping, where you send in a really fast transports to an undefended colony and take it all in one turn.)

DARCA: No, its all to complicated.

I don't think it's much more complicated than the current system. You drop your troops in, they do their thing. You don't want to wait so long? Send more troops! If the battle's not going your way, pull your remaining troops out and redeploy them somewhere else.

Lucky Jack: The current planet invasion mechanic is simply a place holder. GC2 was much more robust.

I don't know about "robust" but there was at least some kind of mechanic. I'm as interested in seeing what they have planned for planetary invasion as anyone, which is why I'm throwing my two cents up on the board here (instead of jealously hoarding them -- my precious! )

While I sympathize with the OP, each turn is  a year. A lot happens in a year.

The comments regarding most of the conflicts you mention is each city is kind of like a planet in Gal Civ. So to win the war, you have to take several, if not all the cities (planets).

I do like the idea of resistance, but in Gal Civ it is a straight up population vs population. You wipe out the planet's population and replace it with your own. I am not saying I like that system. I feel planets should have troops and population. Once you wipe out the troops, you still have to deal with the population.

my 2 cents, which balanced against inflation for when "my 2 cents" was "coined" is worth about .000000001 cent now.

@blaze of glory. A turn is a week.

Darca

Oops, must have been thinking of another game. Ok, then stages! It shouldn't be just planetary invasion, it should be all battles!

<counting down at work until the weekend....>

That is kind of where I was going in my post above. It seems like it violates the lore of the game. After all, if you wipe out the existing population and replace it with yours, where are the Drengin slaves?