Two things struck me that could be enhancements to a zettelkasten program/process for some people for some usages.
Coloured or labelled links like spider diagrams or mind maps.
A limitation is that it imposes a consistency in the use of the concepts behind the links.
Doesn't suit my approach but would really help some.
Tiny notes attached to other notes.
I use stickies attached to documents, but the method is irrelevant.
These mostly arise from subconscious thinking, or something read in passing, rather than deliberate cogitation. Small thoughts, nothing complicated, so if I'm developing a character it might be 'Douglas Firbrae' or 'red hair'; I'm not going to actively think about it at the time - probably actively working on something else - but I don't want to lose the idea and I need it to be where I need it when I do actively work on the topic.
Some tiny ideas, of course, deserve notes of their own. I recently thought of Edward Siwardsson. No time to work it up, but worth retaining. My subconscious may decide to play around with it. Or not. (fwiw, very little is known about him, but he was an Anglo-Saxon/Northumbrian nobleman at a time when the development of Scotland and northern England was being contested between the Gaels, Picts, Irish, Danes, Norse, Anglo-Saxons, British, Normans - some more directly involved than others. He won the battle which was pivotal in giving David I the throne of Scotland; David brought to Scotland the Norman knights whose descendants would later fight for the throne (the Bruces, Stewarts, Balliols, Comyns etc)).
You can see that issues around tiny ideas and snippets are still unresolved in my mind. I'm assuming that Luhmann never had any need to manage them because his thoughts were always longer and more complex. But many of mine are small. They just pop into my head when my brain is unfocused. I have always assumed that this is normal.