Pirates would sort-of be a faction, with effects similar to the others in planet population.
Diplomatic stances should be at least alliance, truce and at war.
Any negotiations besides truce are severely penalzed or impossible while at war.
Alliances should give bonuses, especially economical. I should have to weigh the advantages of leaving my neighbour alive and improve my income by doing commerce with him.
Culture should be a pie divided among all playing factions, plus the pirates sort-of race, a 'corruption' slice which send money into the nether, and 'revolutionists', which could make a planet revolt to become independent (neutral).
You make 100% tax income of the portion of the population affiliated with your race, and initially 0% of the tax income of the other races in your controlled planet. Taxing aliens may improve with research. Research + good diplomatic relations will allow money to flow in from planets now owned by you.
The 'culture pie' can show up on the portrait space in the interface, which is currently under-used.
IF there is a truce or alliance, part of the money generated by the planet would be sent to the ally's coffers (proportional to the culture). Alliances should be much better at it. Also, this might be dependent on technologies, and be improved by them. Like a 'civilian spaceflights' to improve initial trade income, then 'commercial treaty' to improve further the relation, and so on.
Enemy ships in the gravity well should harm culture spread. It should depend on the relative strength to the planet's defenses.
Planet colonization - The colony ship should be maybe a bit cheaper; more frail, faster, take up part of the population in the gravwell it is built on and be consumed when it colonizes a planet or asteroid, just to make the dynamics (and motherships for all races) more interesting.
A planet should be conquerable in different ways. Currently, it bombs the population into oblivion. This should be the worst, most violent way to occupy it. A heavy fallout from the attacks should last for several minutes before the planet can reach its maximum production level.
It should also be possible to occupy it using infantry. much of an enemy population would still be there and would resist, so it would become a cultural warfare to keep control of the planet. My suggestion is to consume the siege ship, or to have a skill with a long cooldown to send infantry to the planet. Infantry attacks would kill part of the enemy population, and another part just defects to your side to avoid being killed. After sufficient attacks are shed on the planet, land resistance would give in and a government would be established by a diplomatic ship. Maybe the 'send infantry' skill could be a researcheable military technology.
The siege ship 'send infantry' skill would also be a military way to counter overwhelming alien culture on owned planets. You just send you military to kill the civilians who oppose your governemt. This will have temporary negative impact on the planet economy. Consuming the siege ship on skill use is a way to avoid having it being abused. If needed, having a population cost for creating the siege ship could be done too - with a temporary negative impact on popularity (reducing how fast your culture takes over alien culture - maybe even to negative values).
Siege ships should also be much more dependent on the military technology on the siege branch, or have a cruiser big brother. This would help to avoid making them too strong at the beginning of the game, and/or useless at the end.
Say the siege branch improves not only damage, but also hull and shields. The initial ship would be the paper craft it currently is, but would get sturdier over time too. It should be slow - maybe get slower as the tech tree advances - to avoid siege spams with no support craft.
The hardest, but best way to take over a planet would be by cultural overwhelming. If the largest part of a planet population is in your culture, you can send a diplomatic ship to establish the government. This may spark war if performed on a faction where a truce is in place, or severely deteriorate relations with an ally, there should be ways to negotiate a payback for a planet - which would likely be another planet, an asteroid, or a fraction of your income for a period of time. Of course, you can just give your ally the finger on purpose, to attack him afterwards (sneaky bastard you are).
