When I look at category theory diagrams, I always think that the visual embedding is underutilized. It seems like the diagrams could be enriched with more symbols to encode more information. For example (and probably a bad example) differentiating between automorphisms and identity functions using different symbols on their arrows). Somehow making these diagrams completely self-contained could be potentially interesting, and I wonder what the tradeoffs would be.
Maybe the current formulation of the diagrams is just right for people who use them to do a lot of heavy lifting. It would be interesting to hear some opinions on this.
You might have a look at section 1.39 in "Categories, Allegories", by Freyd and Scedrov. They introduce a language of diagrams and show how common definitions can be represented this way. Not a particularly easy read.
Maybe the current formulation of the diagrams is just right for people who use them to do a lot of heavy lifting. It would be interesting to hear some opinions on this.