Unfortunately, I'm not sure the game does allow you to set every planet to 'extreme'--is that just a function of the element in the MapSetupDefs.xml? If so, I've tried a number of variations and not arrived at 100% extreme worlds. Also, is there a setting for how many special planets (like Serene, Desert, etc.) are in a given map?
It didn't work as of the last version. I haven't messed with it in Crusade yet. The game code doesn't seem to respect its own percentages/ proportions. One issue is that the system generates 'random worlds' outside of the normal 'planetary system' system. Horevmore suggested some ways around that, and they may work now.
I think there is a setting for how many worlds have traits (aside from extreme traits). I'll try to look around for it if I get a chance. FYI: adjusting the proportion of habitable planets is also not straightforward. There are at least two different settings, and I haven't figured out exactly how they interact.
By adding different forms of life, you have even more options for how worlds are to be accessible, which I think is pretty awesome.
Yup, that is my plan. So far, I have "fully aquatic," "ammonia," "methane," "arid," "cold," "low gravity," "high gravity," "cold," "anaerobic," "autotrophic," and "high pressure" as possibilities. I will never be able to implement more than a few of these, but I would like to make the code so that they can be added if someone wants to expand the mod. It would be really nice if you could combine them. Then again, it would be nice if you could combine planet traits easily too. Maybe in GCIV...
Essentially, all you'll have to do is add a of the RaceTrait (if you want only certain race types to be able to colonize it) and/or tech to the planet and then add any number of to change how usable the planet is based on tech, race type, or other prerequisites. I was fiddling with this in the hopes of making some new extreme worlds, and was surprised by how simple it is in the xml, overall.
I was going to use the RaceTrait option at first, but it was limiting options. My plan is to go the tech route, and have a different tech tree for each RaceTrait. The disadvantage there is having to do more work EVERY time the tech tree changes. The great thing is that, the way the system is designed, you should ("should") be able to do both. I haven't done a test-run of either, and I await your results eagerly.
What I'm not sure about is how the AI would handle colonization in such a scenario
THAT is the rub! The mod I am working on could potentially create a patchwork of territories, and some confusion for the AI regarding how to colonize. I have faith that Brad designed a robust AI that can handle the challenge, but we shall have to see. Again, I eagerly await your results!
For now, I am desperately combing the literature for actual, honest-to-god estimates of the parameters I need to make the mod semi-accurate. Even with the huge Kepler dataset, it is hard to get information due to biases in the data, and a general interest in finding a new Earth. Also, the data are coming in too fast. If I could wait ten years, It would be MUCH simpler!