Feature summary:
https://www.mediawiki.org/wiki/API:Watch says that expiry of a watchlist item may be set, and plenty of builtin pagegenerators may be used to edit watchlist. Both should be used by APISite.watch() and expiry should also be used by Page.watch().

Benefits (why should this be implemented?):
A much more flexible watchlist editing possibility. See https://www.mediawiki.org/wiki/Manual:Pywikibot/Cookbook/Working_with_your_watchlist for ideas.