Some of these proposals might be impractical for a plugin to implement, as I’ve approached most of this thinking from a theoretical side more than a software practical side. However, assuming the queries can be made reasonably fast (either in place or through caching), I don’t think anything here is unfeasible.
This has some similarities to the already richly supported properties in logseq. I would suggest that making it possible to create rich relationships in line in one’s writing makes a huge difference (when I tried to use properties to do this, I ended up writing a lot of my notes in the properties, making my notes look more like a database table than an outliner or writing tool). I’m happy to discuss this more as I think the difference between this proposed functionality and properties is a subtle but important one.
I’m not sure how logseq is planning to build their stated world knowledge graph, but this seems like a reasonable way to begin letting users add structure to their personal knowledge graphs that could eventually be integrated with other knowledge graphs.