"It should support data access for all time." - This is key for me after I had to convert my notes more than once between formats after the original app(s) went into extinction (beloved Circus Ponies Notebook).

That's why I'm designing any new apps around a file format that can be accessed even without the app.

I have a "local-first" Kanban/Trello-style app, "Boards" (http://kitestack.com/boards/), that uses zipped HTML files (to support rich text with images). No collaboration and cross-device support just yet, but it works without a network and saves everything locally.