Is it possible to add the planet Carna? It’s a very interesting icy world with its own enemies. I could use gear from the Better Aquilo lithium battery on it. Plus, its position in space looks very strange when you have the asteroid belt installed. I’d like to have the icy worlds grouped together.

There are currently no plans to extend the number of planets for a while.

The reason is because I'm still missing like 2 smaller "content blocks" for what I already have, and adding an entirely new planet will only make that harder to handle.
Even if I use this for experimentation I still try to be careful with what I do.

The current "expansion plan" is to have an another mod that is similar in scope to the outer rim, doing it's own thing hopefully better than the outer rim does. There are currently two of those on the table in the pipeline. Just harder, because I'm trying to create connections that at least try to make sense.

I know roughly what needs to go into it, but I don't have the exact bits. I also don't want to just throw random stuff at it.

This is going to be my answer. Not quite the thing you wanted, but I think this is better.
https://mods.factorio.com/mod/distant-carna