One idea I had a while ago was to start writing lists of the various parameters that can be found in Spore's PROP files with a description of what those parameters do. This was partially motivated by me not being able to find info like this anywhere. To this effect, I created a wiki with the aim of making these lists available online. I have already added a little bit of data to it so far. I have been dabbling a bit in Spore modding since the start of the year and I plan to continue adding to this wiki as I discover more things.
I would greatly appreciate it if anyone that knows can tell me where I can find any additional information on those PROP file parameters that I can add to the wiki. Alternatively, if anyone that knows what any parameter does, you are welcome to add it to the wiki yourself. I have this wiki stored on four websites. To edit or add pages, you just need a SourceForge, GitHub or Bitbucket account.
I also have this wiki stored on GitLab but, unlike the three above, I don't see a way of giving anyone editing access to the wiki without giving them access to the entire repository. Until this changes, this will be a read-only archive of the above three: gitlab.com/XJDHDR/xjdhdr-random-code/wikis/Spore%20modding
Finally, I just want to be clear that, as much as I would love it, I don't expect my wiki to ever record anywhere remotely close to a description of every parameter. This might possibly have happened if I started this project close to Spore's release but not now, a decade later. As for longevity, I don't plan on ever deleting whatever gets added to the wiki. The wiki is currently stored in 5 places (the 4 websites above and a personal server that syncs the contents of those wikis) so the wiki will still be there if one of those services disappear. Finally, the wiki is a Git repository so the entire repository can be copied and uploaded to another Git service.