I recently thought about what I'm missing, and came up with something rather complicated (sorry, nothing simple haha)
Basically I'm often compiling information in markdown text files. But images are still web hosted. Sometimes I use spreadsheets and wish I could do simple additions in markdown tables.
Or I use bookmarks, but know websites will go down and wish I could just "backup" any website in a compact file and organize it without mucking around with files. Zotero does this partly. I also use "SingleFile" to save websites but it would be nice if it was something semi-compatible to web-archive. Or maybe just save a stripped down version of an article like you see in "reader view" on Firefox. And of course usable in both desktop and mobile.
I do think Project Jupyter does something like this and there are also other note taking apps.
Overall the state of computer science in 2025 is still deplorable. The very basics of storing data still don't have good solutions.