In dark avatar you will only be able to colonize certain planets after having researched certain techs. Now this is a great idea (the sort of thing I lobbied for) bt I fear it will cause some annoying problems in games.

What if another race gets the tech before you and colonizes a planet right in your backyard (in the annoying way that they would colonize the class 4 planet in your home system before you would). This could be really annoying and lead to some rather odd disributions of races through the galaxy. Surely races would "claim" planets in a system, even if they couldn't colonize them.

But this is more of an indication of a bigger overall problem. This system feels a little too tacked on. I don't think it should be a matter of "either you can colonize it or you can't." Furthermore, it doesn't take into account the natural abilities and evolution of races.

*stop here if you don't like long posts*

A far more powerful and interesting idea would be to simply split the idea of planet class into two variables: habitability (how suited the planet is to support your race) and capacity (how many upgradeable squares there are).

The maximum potential capacity is determined by the size of the planet. Then you subtract squares which are water covered, have high volcanic activity, super dense jungle, or are in extremely cold areas.
Some racial abilities would alter the potential capacity. For example, an amphibious race can convert one water covered tile into a useable one. After having researched the Geological Stabilization tech, one volcanic tile would become useable etc.

The habitability determines both the maximum population and the rate of population growth (population grows at a decreasing rate, forming an asymptote at a point proportional to habitability). However, if your food output is even lower, then that will limit population instead. You also incurr higher maintenance costs per unit population for planets with low habitability, so some planets end up costing more to maintain than they bring in, explaining why we don't colonize mars (you'd get a warning before colonizing such a planet).
Some things which would affect the habitability would be atmosphere, with oxygen being best (but traits and technologies might make others just as good) or gravity (earth sized would be best, but races with the large/small traits would receive reduced penalties for large/small planets).

But the point is, all planets would be colonizable from the start (except gas giants, though perhaps there could be a tech that allowed cloud base style settlements). Some might be valid only as sensor outposts. Some might be unprofitable, but they might have unique features like large research bonuses.

I feel something like this would be a far more versatile, balanced and powerful system. It could also allow for some really interesting stuff within it's scope. A planet with agressive native lifeforms (mindworms!) receives a habitability penalty and buildings might face the risk of discussion. A planet orbitting a binary star system or with numerous moons would have more volcanic squares due to the varying graviational forces.
A lot of stuff like this is already part of the game, but it is only as "one off events," rather than integrated into the system, which is almost certainly preferable.

Additonally, this system would allow the game to look at your race's abilities when generating your homeworld, so that it looks like the kind of place they would have evolved on. The Arceans would get a large heavy gravity world, Drengis might be volcanic etc. And it doesn't need to be complicated for new players; from these two values, you can derive whether it's a good planet to colonize, and stick a smiley face over it (or whatever).

Thoughts?