Original description was as follows:

Author: robchur

Description:
Users on En. are requesting the ability to add comments to watchlisted items, for instance, to remind them about why they watched the item in the first place, or to "tag" certain items, e.g. "featured", "vandal magnet", etc.

While I realise this might need an interface overhaul, I've also — literally this second — thought of a fairly flexible way of doing it.
Anyway, here's the feature request, just so's I can assign it to myself when the time comes. ;-)

Version: unspecified
Severity: enhancement
URL: http://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_%28technical%29&oldid=32356168#Watching_pages

The OP above didn't specify how this might work. Here's an idea that might:

It's common practice in computer languages to use a hash symbol # to preface comments. From that symbol to the end of the line is comment, but the next line is normal again. That symbol cannot be used in the title of any wikipage. Since the watchlist is listed line-by-line, we could have a basic set-up of something like:

Canada #Countries
United States #Countries
Lake Michigan #Lakes
New York City #Countries

Watchlist entries might have many such comments:

Canada #Countries
United States #Countries
Lake Superior #Lakes #USA #Canada
New York City #Countries #USA

For multi-line comments, we might think about using square brackets, i.e. [] which are also not allowed in page names. They could be used to comment out whole sections of code.

This does hint at way of categorising watchlist entries. Then users might be able to filter their own watchlists by hash-category. Generative AI could easily take a list of watchlist entries and add tags to these, so if I only want to look at edits to #Countries

You might also need something like HotCat to the page, where comments could be added/removed from a watchlisted page.

Watchlists are a pain when they get too big, which they have a tendency to do, despite "expiring watchlists."