I try to think of game mechanics in a very tabletop/boardgame way. Your above idea of "culture" really intrigues me and I came up with this:

In many tabletop RPGs your character might obtain bonuses or "perks" that allow him/her to more easily hit or break an enemies' defense during combat. What if Culture was a measure of a planet's ability to "resist" the colonization of the Empire? For instance, a very high culture on Planet Gorp provides bonuses to the defenders' offensive capability and the population's resistance to adopting the Empire's rules (civil unrest, terrorism, coups).

BUT... if you can manage to spread the Empire's culture to a planet (spies, propaganda, space missionaries, entertainment) you can mitigate the above and turn the situation to your favor with penalties to the planets defense.

In a tabletop game this could be represented by a simple Attribute that influences various in-game tests:

ATTACK = (Weapon Skill + Weapon Rating + Random Roll) +/- Culture Bonus/Penalty

I hope this help! Realize it is a half-baked idea being typed at 5:51am. Adapt it, change it, improve it, or junk it!

But what you think of these neutral planets in general? Do these fit? Maybe remove them?

The thing is, if I increase the number of planets (since these have no buildings, just one industry type - so we can afford to make more planets without overexerting the player) I will have more neutral planets, yet there is a limit to different races I can introduce (it's not even about implementation/budget, simply having more than 10-20 races is too much to remember/grasp to a player...) Yet, with this size of galaxy (like 100 planets let's say) making 10 neutral planets with unique races makes no sense to me...

I'm not sure.

But what you think of these neutral planets in general? Do these fit? Maybe remove them?

Neutral planets are a fitting addition. If I'm grasping the whole of your concept, they represent a "resource" that is not so much contested but unaligned. This certainly can lead to competition to control that resource that is different than finding unoccupied planets or colonizing enemy planets.

As I reflect on your game I can't help but to think of the Crusades, particularly the First through Third. I suggest you read some sort of brief history of the ebb and flow of that sad conflict for some ideas. The Islamic world shared a common faith, but they were often at odds with each other, allowing invading armies to conquer in the face of daunting odds. It seems to exemplify the idea of how neutral planets (in this case cities) relate to the surrounding power struggle.

Do races move ?

Are there racial differences ?
Will planets be unique enough without a unique race ?

The low quantity of these really bothers me... I simply can't make 40 different races (which would make 40% of planets neutral which would be the best in my opinion).

Neutral planets are a fitting addition. If I'm grasping the whole of your concept, they represent a "resource" that is not so much contested but unaligned

This certainly can lead to competition to control that resource

As I reflect on your game I can't help but to think of the Crusades, particularly the First through Third. I suggest you read some sort of brief history of the ebb and flow of that sad conflict for some ideas. The Islamic world shared a common faith, but they were often at odds with each other, allowing invading armies to conquer in the face of daunting odds. It seems to exemplify the idea of how neutral planets (in this case cities) relate to the surrounding power struggle.

Yes, it's kind of the same here, the player is the Empire, all planets under one rule. Neutrals are divided.

But, more importantly, in the log run the neutrals will have to join to empire if they want to survive (invasion of aliens from another dimension that want to annihilate everyone). Also, neutrals share the same enemies with the empire.

Competition with whom? There are no other empires.

Who/what-ever you are fighting -- I'm using the words "competition" and "resource" in a super vague way. The planet not wanting to join the empire is competition. Or, as I tend to look at these games, any spot on the map, no matter how small, that I don't control is an affront and my competition!

Maybe like this:

There are 8 races in the galaxy (+ some hostile/toxic ones: borg, symbiots, aliens from the "Alien"). One of the races (Terrans) is the founder of the empire (player have their allegiance and their homeworld).

The player does not colonize planets (or not only the player), each race (doesn't matter if they are part of the empire or not) auto colonizes planets nearby their homeplanet from time to time. These planets are considered neutral/independent (in some cases they will instantly join the empire). The Empire can persuade a single planet (not the whole race) to join the Empire (especially easy if within cultural zone of the Empire). Upon first planet of that race joining the Empire you select the status of that race within empire (slaves-equal-masters), it affects all further planets of that race (well, it you set them as slaves they might not be so eager to join :D), if the homeworld of that race joins the Empire you get +50% to all other planets of that race joining the Empire.

Althrough, I'm not sure what should happen if you attack a planet of that race (does other planets will help them?). Note I would prefer to not implement any sophisticated algorithms for neutrals, since that's not the scope of the game.

Rebels - a planet can secceed from the Empire (event, some conditions, poor imperial stability value, etc). Such planet becomes a rebel (semi hostile), and can (and should) be reassimilated by the Empire. I think rebels should fight the empire actively somehow.

You can "grant independence" to ALL rebels planets (can't grant it to just one), in such case they will stop being rebels and become neutrals, they get tons of bonuses against joining the empire (cultural assimilation) and you are not allowed to conquer them for 100 turns (or face big prestige drop). Also, you lose some prestige (sometimes letting them secceed is simply not an option because it will destroy the empire internally).

I need help on these races/neutral planets. Not sure what model would make sense...

- make the galaxy empty (no neutrals only hostiles) and only one race (Terrans) that conlonizes it?

- make 8 races and they auto colonize regardless if they belong to empire or not and planets act separately (not per race)

- make like 40 races (a bit insane :D), problayby just parametrized (like: Race A is scientific + aggressive)

Or some other model?