The idea of an index is to provide launch-pads to start exploring the various ideas and connections in your vault. So the basic structure is listing central keywords, and providing one or two notes that are central to that keyword. It’s not meant to be an overview of the vault. And all permanent notes should link directly or indirectly to the index. This is how I do it, at least, and it’s only meant as guidelines, not rules.
I forgot to mention that I’m using a pretty standard Zettelkasten setup in my library.