As the title asks. I'd like to make this place the go-to libre culture forum, since so many others are dead.
A definition of libre culture, surely. I was thinking of keeping the rules sparse and broad, the usual "don't be rude" rules, but I was also thinking of adding a list of the kind of posts that are allowed here, such as memes and support requests, to help stimulate activity. If those kinds of posts aren't desired, I can make dedicated communities for those kinds of posts anyways.
Regarding a definition, I'd try to look for a succinct one, as longer text in the sidebar tends to look busy and overwhelming. Maybe alongside it a few examples of libre projects/media, say stuff like Project Gutenberg and LibriVox to creative software like Krita/Godot/Blender/etc. Examples would give a much clearer idea of what's considered libre culture to discuss than a definition alone.
Also I think the idea of clarifying what sort of posts are allowed/desired is good, and speaking of memes, some restraint may be wise to avoid it becoming simply a meme community. Maybe a semi-regular memes thread or something? If said threads were active enough then you would have a better sense of whether or not setting up another community dedicated to them is worth it.