Hmm... good thing I did some thinking on my spare time. I think you guys will like this.
Invasion: The new player ability allows players to take eachother's planets hostage without bombardment. The player who's planet has been invaded can bomb his own planet but at a large penalty to culture on the same planet and a small but permanent allegiance loss on each planet. When a player invades a planet he gets to keep whatever planetary upgrades that were researched, and now controls the gravity well.
Ground Control (GC): This serves as the buffer between invasion forces, and ultimately determines who wins a land battle. This new ship resource serves to represent a ships capability of fighting invasion forces. GC can repel hostile culture up to twice the regional net of friendly culture against a minimum allegiance.
Planets will still be considerably weakened by enemy culture, but GC makes sure that the planet still retains a minimum allegiance. As previously stated all allegiance below the minimum is up to twice as hard to change as everything above it.
GC is a constant ability that is only in use on the planet whos gravity well the ship in question inhabits.
Every structure in the game has this as well, but only hangars and starbases have a decent amount capable of defending a planet. GC doesnt actually give any benefit to the player using it unless an invasion force is sent down to the planet from the following ship types.
Warship: capital ship that has a large amount of temporary GC points and whos invasion ability allows a large but temporary GC bonus.
War Corvette: lands on the planet to supply a permanent GC bonus up to a maximum of (X). The ability that allows it to do this also is capable of starting an invasion.
Planetary upgrades and research: Planetary upgrades can be researched to give planets a small GC bonus, but any serious defense against GC should be invested in war corvettes and other things. Planetary research can be attained in which ground forces gain an air to space weapon whose power is dependant on the amount of GC.
No matter how immaculate the defenses, a planet always falls to the sheer numbers of fleets. This is no less true with GC points as they apply to fleets. A very large fleet's GC will always be superior to the planets without a friendly fleets help.
Simulated GC animations: A kodiak for example can provide GC realistically because it has a small number of surplus armed personnel onboard, and can also provide planetary laser guided bombardment as air support for friendly forces below where previously its weapons were insufficient without the support of ground forces.
A transporter cruiser also has surplus personnel aboard and can also provide strike craft support. These simulated effects dont infringe the ships dps (damage per second) against enemy fleets at all, but they are simulated for the sake of realism. For example; miniature transporters go to and from the planets from the ships, small vehicles appear on the planets surface, strike craft circle it, and small explosions (smaller than normal planetary bombardment) sometimes appear.
The normal traffic stops due to the planets conflict.
When a planet actually gets invaded, unless there is opposing GC it is instantly subjugated. If there is opposing GC, the allegiance will shift slowly in one direction according to whos GC is greatest. Ten way invasions are possible, as are joint invasions.
The system, while it could do without, would greatly benefit from realtime land battles:
There would be another window which you could expand according to your preference. The window represents a battle on the planets capital.
If computer performance is an issue, land forces could be very simple 3 dimensional polygons made to represent land forces as if you were commanding them through a computer terminal, still maintaining a sense of realism yet alleviating computer performance issues.
According to the amount of GC, certain units could be created similar to ships in which each is useful in their own way.
The battlefield would be decently sized for a 10-20 minute game against the other players ground forces. I can see ten way battles being a problem however... maybe they would still be possible.
Many units are similar to the ones in space. A land equilvalent to a light frigate would be a simple rifleman, a LRM a sniper, a protev a drone that sets up outposts on designated capture points, kodiak a shock trooper in powered armor, capital ship a tank, flak frigate a ATA trooper, a transporter doubling as an APC and drone carrier, and the support cruiser medics and engineers, etc. And no, Im not saying that those units would only be available if you had the corresponding ship type in the gravity well, Im merely making an analogy.