Just some suggestions, though the title mostly serves as a reminder. Here is your requested moddable asteroids.
Simply saving clusters without commands to a specific group ID, most probably 100 since that's where the asteroids blocks are, where the game will draw from to populate the galaxy as asteroids. Perhaps a console command similar to ssave that allows for saving the cluster as asteroids and a section in regions.lua that specifies the blueprints as asteroids. Maybe even include some more parameters like asteroid spawning frequency, density, and region location layer specifically for asteroids that overlaps faction regions.
Once again having group Id 100 as the asteroid, recycle the randomized asteroid generator to use other modded blocks in group Id 100 with the same shape and scale as interchangeable replacements for some of the original asteroid blocks. This can be used for ores and such, but is limited to the vanilla shapes. To advance on this mechanic, maybe a specific lua file, let's call it asteroids.lua, that defines the asteroid characteristics. For example, asteroids.lua can define types of asteroids by specifying the associated group 100 blocks to be used in the asteroid, and letting the already implemented asteroid generator handle constructing each individual asteroid with the predefined block sets. Can overlap with the previous idea and have these asteroid's placement be modded too. However, all the asteroids may start to look the same.
A combination of both.
Plants: If I'm not mistaken, couldn't you already get the block palettes for plants and save your own blueprints? Wouldn't the game would draw from these new plant blueprints without any change to the base game? Or if you mean procedurally generated moddable plants, https://feathub.com/Akaito/reassembly-ai-mod-example/+90
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