Quote Originally Posted by Yora View Post
It's unclaimed space. Anyone can plop down a new colony on any ball of rock of they want to. If the colony is established by a government, that planet is now effectively part of that state. The only thing that keeps other people from colonizing that planet as well are the people already on the planet and the people who have funded the colony.
If people want to start a fight over a planet, they absolutely could. But I see little point in it. I established that calculating new routes to new system is very expensive so that players will have to restrict their journeys to the lines on the map. But compared to the overall costs of establishing a new colony, that's a pretty small factor. And it completely pales compared to the costs of armed conflict. Fighting over dirt is just not economical.
If people do fight over planets, and as said earlier, that's assumed to be a very rare thing, then it's about infrastructure and possibly populations. The dirt is the same as in thousands of other systems. It's what people have build on that dirt that's valuable.
Along these lines there are some things to consider with regard to settlement patterns. Specifically, do you want extractive industry to be focused on planets or asteroids? In high-realism science fiction extractive industries gravitate to asteroids because you have to spend delta-v to get in and out of gravity wells, but if you remove the fuel cost issue people are likely to concentrate their efforts on planets - even uninhabitable ones - because decent gravity, an atmosphere (even one you can't necessarily breathe), and other factors making running said processes technologically easier.

Similarly you also need to think about food production and recycling technologies. Existing human technology has already reached the point where you can feed humans off food entirely grown in vats using artificial illumination (it's not yet economical to do this on Earth save in specialized conditions). Coupled with sewage filtration and recycling technologies this allows extractive colonies on airless rocks to be pretty darn close to self-sufficient even without going the entirely automated route. This kind of thing has a significant impact on the amount of intra-system traffic you get.

That being said, I can see conflict over planetary real estate if 'Garden Worlds' meaning planets that are immediately habitable by one of the sapient species without the need for terraforming or environment suits, are sufficiently rare. This depends on a number of things, especially whether or not you're operating under a panspermia model of biological development (in which case all life shares a LUCA somewhere billions of years in the past) or you have multiple separate abiogenesis events behind your biospheres. Many of the common aspects of 'retro-future' type space opera, such as inter-species romance (even if hybridization is impossible), or just everyone breathing the same air mix, work better if you go the panspermia route.

Another thing to consider is the possibility of factions with non-economic reasons for conflicts over territory. This includes the old standby of 'holy worlds' but also more modern conceptions like eco-purists who wish to protect virgin biospheres from settlement by outsiders and even esoteric ideas like planets claimed as gigantic art projects. And there's always the classic 'world overrun by the biots/robots of the precursors.'