Some things to consider...
None of this is particularly pressing or vital though.
Terraforming options adding volatiles
While this is a very interesting and useful feature, I think it may be a little too "good".
Changing a planet to Frozen generates a volatiles resource with a value of +2, the highest possible.
While this could be explained as uncovering or otherwise making accessible new exploitable areas, this trivialises the presence of volatiles in a system removing part of the balance required to locate a suitable colony site.
There is already a precedent for generating resources "out of nothing" in this mod: Mining stations.
They generate expolitable ore resources with a value of +0. Enough to sell profitably, but not enough to run industry on it's own.
Would suggest that new Frozen worlds generate a volatiles resource with a value of +0 to bring it into line with the already extant Mining stations.
And all other instances of terraforming generating volatiles creates a -1 resource.
(-1 volatiles is still incredibly profitable.)
A sort-of counter to this, would be if you manage to find a means of dynamically generating a resource randomly from a specified range of outcomes. That would alleviate the concerns about always being able to ignore a resource due to being able to create it "on demand".
There's a similar thing with Food and Organics, although I don't really consider that an issue as they are somewhat unique in that neither are impressively profitable, so higher value resources don't make such a pronounced difference.
And both are "industrially" useful even at -1, so thier actual resource concentration is largely meaningless. If they're present at all they're enough.
I see your point about how the resource levels on some of the planet transformations may be too high. The problem is that terraforming costs a lot of time and credits which could be used to simply conquer high-value colonies in the core worlds. The bonus from terraforming has to be high to justify this cost. Plus, many players expect custom-terraformed planets to be superior to most of the randomly generated ones.
With regard to frozen planets, like you said, +0 versus +2 volatiles is not a huge difference once the colony grows to a decent size and has AI cores. Why lower the value and disappoint some players who spent time terraforming their world?
With regard to volatiles on terran planets, I originally didn't have them there, but I was worried that without them terraformed terran planets would always be inferior to terran-eccentric worlds like Eventide that have all four mining resources, and players would be disappointed by this. I could always have both terran and terran-eccentric terraforming options, but terran-eccentric would just be superior in most cases once the colony reaches a large enough size.
Making the resources dynamic would probably be a bad idea because players will be confused why their planet doesn't have the resources they expected it to have after they spent a lot of time terraforming it.
Terraforming options
I would like to see the addition of Arid as an option for terraforming.
While Desert is similar to Arid, it is not the same. Deserts are considered only marginally habitable, while Arid is fully habitable. And it would be nice to have that option available if the player wants a hot dry option for improvement. (Jungles and Oceans being notably un-dry.)
From a technical standpoint, the inclusion of another entry in the selection list shouldn't overly clutter the screen. Although any more than +1 might run that risk.
(RIP Eccentric.)
Another thing to think about is possbily limiting the available terraforming choices based of the current temperature of the planet in question.
So it would not be possible to change a hot world into Frozen or Tundra without first adding shades. (These types cannot exist in vanilla with the hot condition.)
And likewise would not be possible to change cold worlds into Desert or Jungle (or Arid) without first adding mirrors. (These types cannot exist in vanilla with the cold condition.)
The rationale being that the current system doesn't really need the reflectors all that much, as every option is available from every starting point. Which itself leads to oddities such as a Frozen planet in a close orbit of a bright star with the "very cold" condition, but no shades.
Or hot Jungle planets in the outer system with no mirrors.
I would like to see the reflectors play an integral part of the terraforming process as they did in previous versions. (This current verison is a much better system, except for this one part where you've accidentally marginalised one of the coolest features - it's currently easier to use the systemwide options.)
The challenge here would be communicating the requirement for this extra step to the player.
Text on the terraforming options screen would likely be the easiest way.
Although "greying out" locked options with a "you need to build some reflectors" appended to the end of each one possibly being a slicker approach.
The arid transformation has been requested several times and I think I'll replace the desert option with it. I want to use the last slot for the arcology/hive world type that I'm currently working on. Thanks for that texture!
I know that some players are disappointed by the simplicity of the new terraforming system compared to the old one, but overall the complaints about the complexity in the old system were so prevalent that I don't want to reintroduce requirements related to temperature, water, etc. Experienced players may prefer them, but I don't want to create a frustrating experience for first-time players.
Players can always "role play" by building reflectors before starting terraforming if they want to adhere to a higher level of realism. Also, everyone has different ideas of realism. For example, things like the greenhouse effect, albedo, biosphere, etc. can alter temperatures on a planet without using stellar reflectors. If I require reflectors for certain transformations, I will have people complaining about how it would be possible to do the transformation without the reflectors. It would be way too much work relative to the benefits to create multiple different terraforming systems (and the accompanying UI logic to grey things out) to satisfy everyone.
This mod has come a long way. And even as-is is incredibly impressive.
Trully excellent work!
You've been one of the biggest contributors of feedback and art assets since the beginning of this mod - thank you for all your help!