I agree, and this could be done in the following way:
- all tiles on the planet are given a specific type (desert, ocean, artic, radioactive and so on)
- all races would have one or more prefered tileset
- a given race would see a planet as habitable depending on it's preferences and would not necessarily use the tiles that another race would use
- this would mean that a planet habitability would not be the same for all races (some balancing required here to generate the map, but I believe nothing unfeasible) ; also, for convenience, planets could be broadly categorized as barren, artic etc. using that classification to generate the appropriate tiles (more tundra and artic on artic planets etc.).
- technology could unlock some (all ?) tiles as terraforaming (and maybe other) techs progress.
For example, a planet could be "moslty" arctic. Meaning when you first land there as a "normal" race, you'd have 2-3 available tiles (likely the equator...). Another race ("artic" race) could see the same planet as beeing quality 20, that planet being well suited for them. A weather control tech (which would have to be built anyway) could then later unlock the arctic spaces and leave them open for terraforming (i.e. they would still have to be individually converted).
Does not strike me as very difficult to implement (most of the mechanism seems already in place), but I may be mistaken.
In addition, I think that most planets should be at least 2 in habitability: 1 for the basic colony, 1 for the appropriate terraforming equipment. And basic, underdevelopped colonies, should drain your treasury.
Then, there could be a classification based on the atmosphere type (oxygen-nitrogen, nitrogen, carbon dioxide, methan, none...) ; a planet would be mostly uninhabitable under the wrong type of atmosphere (but good for you if the right type). Under these conditions, only domed cities could survive (if you're the wrong type), which would actually mean very low habitability. The 2 above: as long as you do not have built the appropriate atmosphere converter (generator) you'd be stuck with what basically is an outpost. Good for the long run, a drain in the short view of things. The same could be done with gravity.
Such a mechanism would greatly improve the game : as of now, the problem is to grab the best planets around, and that's pretty all: further expansion means war. With such a mechanism, you could attempt to tread a peacefull expansion path by colonizing the planets that nobody wants. You would also always be faced with the question of whether you expand your outpost number (costly) or just hope nobody grabs empty planets before you claim them (instead of the current strategy where there is an initial rush, then nothing).
Yves