Thought about this some more, and I'd like to expand on it. The goal is to make colonies more interesting, and make searching for terraforming techs a big part of exploration.
So, the following values are created for each planet during generation. Each has a base value and a modified value, with the modified value being what you actually can see (perhaps a skill might let you figure out the base values during surveys). Raises means as this stat goes up, the other goes up. Raised by means the opposite, when the other goes up, this goes up. They apply to the modified value only.
Mass (Sets gravity level. Raises Atmospheric Density and Tectonic Activity. At very high levels, raises Radiation)
Radiation (Solar winds and radiation. Affected by type of star. Raised by proximity to star, Lowers Atmospheric Density. Raises temperature)
Atmospheric Density (Air pressure, basically. Lowered by Radiation, raised by Mass. Lowers accessibility.)
Atmospheric Toxicity (Amount of dangerous stuff in the atmosphere. Lowered by low atmospheric density.)
Temperature (Average temperature of the planet. Affected by type of star. Raised by proximity to star, tectonic activity, and radiation. At high levels, lowers water)
Tectonic Activity (How active the core of the planet is. Raised by proximity to star, or in the case of moons by proximity to any very massive planets. Raises heat, lowers radiation).
Moisture (Overall amount of water. Lowered by very high heat)
Now, existing planet types are defined, on generation, by these values. For example, a world that's got high tectonic activity but low temperature is likely to be cryovolcanic. This makes for easy generation of worlds (and we may find there's a few gaps where we want new world types, plus modders can create new world types that fit in this system).
But what advantage does all this give? It allows for interesting terraforming. Search the stars, find terraforming technologies, and build them (using nanoforges) on worlds. For example, a solar shade reduces heat a lot and radiation a little, but a planetary shield lowers radiation a great deal and heat a little. Solar mirrors increase heat a lot and radiation a little. A water exporter lowers moisture a lot, but makes that water available to a water importer which raises it a lot somewhere else. All of these cost industry slots and maintainance, so while in theory you could get a lot of such techs and turn terrible worlds amazing, it might cost too much to do so for many of them.
Plus there are chaining effects. A radiated world that gets an ionosphere magnetizer and planetary shield will have its atmospheric density shoot way up, perhaps revealing a far more habitable world... or one with a highly toxic atmosphere. Skills could be used to figure out what that base toxicity is before deciding to do that.
As an additional idea, energy could be required to power both industries and terraforming projects. And energy sources could depend on worlds. Geothermal power plants would be very effective on highly tectonically active worlds, while higher radiation levels improve solar power networks. Floating solar arrays could both reduce radiation and use the unmodified radiation level for power generation. And so on.
The overall result is that each colony will have entirely different build paths depending on the world it's built around, and exploring for cool terraforming technologies heavily changes where and what you can build.