I’ve thought about this quite a bit! It seems like this would effectively be a desktop alternative to highlighter. I actually don’t know of any good desktop long-form event clients with highlighting. Maybe they exist?

Part of the reason I prefer Obsidian strongly to something like notion or web-based apps is that it is local-first (both the notes and the interface). This allows for a more responsive interface, local interaction with notes outside of the interface in other software, flexible back ups, and better privacy.

One of the most interesting aspects of nostr is that the relays are sort of optional. They do have first class support in the protocol, but the cryptographic proofs make it so that we can all agree an event is valid whether it is on your hard drive or on a relay. Some relays have limits on timestamps they will accept, but otherwise there is nothing stopping a long-form notes desktop app from being offline first, with secondary relay publishing as an option and potentially even a peer to peer sync if you wanted to set it up via a plug-in.