Do have a link to all the single html file apps you have in mind?
The two projects out in the wild that natively work with this approach are TiddlyWiki and FeatherWiki.
I see room for a lightweight version of a calendar, a world clock, and even a lightweight spreadsheet that could be useful. I also have an idea for something I call a link trap where you can rapidly just drop links in and then search and sort over them to quickly retrieve something you saw before that was interesting. Sort of like my bookmarks-outside-the-browser except more of a history-outside-the-browser.
My primary saver is a Python script that I wrote called Notedeck, but I also sometimes use a Rust webdav server called dufs.
I haven't released either of my projects I'm working on that are the client files, otherwise I would have just linked them.