A defining feature of space is its vastness: any number of starships, with several orders of magnitudes of margin beyond the expected maximum number of ships in the whole game, should be able to occupy a map sector without being engaged in combat.

Combined with the preference for "starlanes" (suggesting that everyone hangs around their endpoints as much as possible) and the desire for protracted combat, the possibility of coexisting with the enemy in the same location suggests rules in which engaging the enemy is an optional and often deliberately delayed action, and choosing where to put reinforcements and who/what deserves attacking are the main strategic elements.

• Every map sector contains any number of mobile fleets and stationary planets and stations of any faction. Newly built ships join a fleet in their sector at the end of the turn and can move the following turn.
  Sector size can vary from most empty and a few with one planet or station, to planetary neighbourhoods encompassing satellites and Lagrangian points, to whole solar systems; something important to vary and tweak for tactical and strategic purposes.
• Movement speed: In a turn, a fleet can attack any enemy fleet in the same sector (or merge with a friendly one), or go to a planet or station. Fleets can split freely at the end of the turn (to show the resulting new fleets when the enemy selects moves next turn).
• Fleet combat takes place after movement; it can be assumed that all fleets who go to the same place or approach the same fleet arrive together. With suitable cultural excuses (prudence, chivalry, expending limited fuel or ammo, etc.) the normal outcome is that both factions retreat after modest damage, it doesn't matter where. Next turn they'll go anywhere in the sector all over again.
• When a fleet goes to an enemy planet or station, they first engage enemy fleets who went there (and those who went after them); if attackers are successful enough, as an exception to the rule of always retreating, they remain, forcing the defenders away, and attack the objective in the same turn.
• "Long-distance" movement: a fleet shouldn't go further than to an adjacent sector in a turn, and only if no enemy fleet attacks them. On the following turn, they are in the other sector.
  Spending additional turns in wormholes, warp speed, or whatever you want to call states in which the fleet is traveling and cannot attack or be attacked can be an option.
  Slow movement enables bluffs; fast movement wouldn't require a player to commit its forces to a specific place. Slow movement on a large map can be made more agile with some kind of shortcut between ordinarily distant sectors.

Beautiful...

I can't use it for my project (no sectors, just planets with starlanes) but I can't resist pursuing this concept :D I always loved sectors.

To rephrase what you wrote. The map is divided into squares, each consisting of several planets/systems/asteroid belts/satelites/orbital basaes/etc. Fleets are placed in a sector, then they take a position on one spacial body they occupy. There are no disctances within a sector, all fleets can engange any unit within system.

The problem I see is stopping enemies. Like, if you move a fleet to a sector (just moe them into space, no planet targetted), then enemy targets your fleet, but you gave an order to move to the next sector, so at the end of turn the enemy has invalid target (you are not there anymore) and you moved to another sector, effectivelly passing through the enemy lines.

A suggestion: look at Sins of a Solar Empire. Map layouts have star lanes that connect certain planets to others (you have to pass through some planets to get to others). You get ships you can direct individually, so it isn't one big fleet. The more planets/resources you have, the better position you are in. Eventually it is about elimination/surrender. Retreating is always possible and planetary reinforcments let you prevent/delay retreating.

I can't use it for my project (no sectors, just planets with starlanes) but I can't resist pursuing this concept I always loved sectors.

Each planet neighbourhood can be a map sector; if they are too complex they can be split into a few parts (Solar system example: assuming extensive colonization making Moon interesting, treat Earth and Moon as two adjacent sectors rather than one).

With a setup of planets and "starlanes" you simply don't have many void map sectors because nobody goes too far from planets; void sectors might be present in strategically important locations within a star system (e.g. the Solar System could have sectors for each planet, important satellites and asteroids, the sun and two empty ones "above" and "below" the orbital planes).

To rephrase what you wrote. The map is divided into squares, each consisting of several planets/systems/asteroid belts/satelites/orbital basaes/etc. Fleets are placed in a sector, then they take a position on one spacial body they occupy. There are no disctances within a sector, all fleets can engange any unit within system.

No, not squares. Sector shapes are irrelevant and effectively nonexistent, sector sizes are vague and easily fudged, and particularly with arbitrary long-range links, the graph structure of sector adjacency is going to be arbitrary and likely not planar.

The problem I see is stopping enemies. Like, if you move a fleet to a sector (just moe them into space, no planet targetted), then enemy targets your fleet, but you gave an order to move to the next sector, so at the end of turn the enemy has invalid target (you are not there anymore) and you moved to another sector, effectivelly passing through the enemy lines.

Actually, I suggested that a fleet can only exit a sector if nobody attacks them; I'm afraid I didn't make the assumption of simultaneous turns (orders for each fleet, battles of fleets attacking each other or converging to a location, planetary assaults, and finally splitting and coalescing surviving fleets) sufficiently clear.

If your fleet wants to stop an enemy fleet and they are in the same sector, just attack them every turn and they won't go anywhere, except perhaps taking the battle to planets or stations of that sector they want to conquer or hold.

With a setup of planets and "starlanes" you simply don't have many void map sectors because nobody goes too far from planets; void sectors might be present in strategically important locations within a star system (e.g. the Solar System could have sectors for each planet, important satellites and asteroids, the sun and two empty ones "above" and "below" the orbital planes).

In my setup each system has just one planet (system=planet), it simplifies soo many things :)

I'm not sure what you meant by "you simply don't have many void map sectors because nobody goes too far from planets"?

I'm not sure what you meant by "you simply don't have many void map sectors because nobody goes too far from planets"?

A sector without planets or stations is only important as an intermediate location along the path between important places, and if you have "starlanes" most travel will not involve the empty space between planets or stars, so adding huge numbers of empty map sectors to represent it is a confusing and pointless complication. The only empty sectors should be the ones that can be reasonably travelled through and the ones that adjust the topology of the sector graph (for example, what sectors need to be occupied to blockade what other sectors).