Right now there’s very few visualizations available on Plexamp (for Windows at least, which is what I use most often vs the Android one) - but one of my favorites is just the simple VU Meter from foobar2000.
If you could allow users to write their own visualization plugins and then sideload them into a directory for Plexamp or something, that would probably be best - like the glory days of winamp, personal choice in visualizations and visualizers could reign supreme.
Bonus points for making a curated store or github that contains curated visualizations that can be upvoted for popularity, but that’s a different can of worms.
I would love to have old friends like Milkdrop back. I confess that I find the built in Plexamp visualizers and their palette choices miserable. Everything seems so… beige.