It would be nice to be able to add non-public notes for oneself on interesting questions on MO (of which there are very many), for later remembrance and further thinking on it. These notes can remain as part of an individual user's profile. Can this feature be added to MO? In my opinion this should be very helpful (a version of a work diary for users). If the notes themselves can be hyper-linked, it can become a non-linear work diary and aid thought on a problem or a topic.
As discussed in the comments, this isn't exactly what you're looking for, but it is an existing functionality (since October 2022), so it is probably worth mentioning.
You can use saves to mark questions you're interested in. You can organize saved questions into various list and for each saved question you can leave a private note of length up to 500 characters.
I will stress that the saves are private - they're visible only to you. There is a related feature request: Allow setting public visibility settings on saves lists, such as "public" and "unlisted". And, as you mentioned in a comment, there isn't a hyperlink to an individual save. (But each of the lists has an individual URL.)
You can find a more detailed information on Meta Stack Exchange: Bookmarks have evolved into Saves and the corresponding tag-info. Saves have been briefly mentioned on this meta, too: Any chance of a "save for later / TODO" feature?
You can also have a look at various similar feature request on Meta Stack Exchange or on various per-site-metas - just to see what was recommended there. For example:
- Adding a note to a favourite
- Private notes on questions?
- A note making feature in the personal page
- Private tags or other private searchable annotations
- Favorites improvements - search, categorize, personal tags, add note, favorite answer
- Favourites Enhancer: Tag favourites and add personal notes on Stack Apps
- private notes about a question (on Mathematics Meta)
- Is here something like blog service or just sand-box to store notes ? (on MathOverflow Meta)
There are browser extensions that let you add notes to any web page. This could be a workaround.