I can request a list of recent changes for items from my watchlist:
https://en.m.wikipedia.org/wiki/Special:ApiSandbox?useformat=desktop#action=query&list=watchlist&format=json
generated from articles in my raw watchlist - https://en.m.wikipedia.org/wiki/Special:ApiSandbox?useformat=desktop#action=query&list=watchlistraw&format=json
Logically it seems that you might be able to use a generator to swap out the pages used to generate the watchlist e.g. rather than watchlistraw pass in a list of pages in a given category (for example https://en.m.wikipedia.org/wiki/Special:ApiSandbox?useformat=desktop#action=query&list=categorymembers&format=json&cmtitle=Category%3ABirds) to show edits across them.
This would be extremely useful to get delivered in a single API request allowing us to build all sorts of web apps and explore multiple watchlist type functionality.