What is the "wot:" doing? Couldn't it just be context?

But yes, the single letter is just indexed. In theory, all tag names should be searchable. I havent checked support for it though.

I was thinking on doing indexes for the presence of things in a similar way NIP-32 does with namespaces.

We would just do ["rank", "90"] and then a ["w", "rank"] if the goal is to search of all records that have rank.

We could also do multiple precisios, like geohashes, ["rank", "9x"] to download all 90s ranks.