Problem: I want every page I edit to be added to my watchlist for a limited time, so I can see what kinds of follow-up changes other users may make in reaction to my edit. Because my edits are generally maintenance-related in nature, I edit a wide range of pages, and have no interest in edits that are made months or years later.
Proposed solution: In the Watchlist page of preferences, provide an expiry dropdown next to "Add pages and files I edit to my watchlist", "Add pages and files I move to my watchlist", and each subsequent checkbox in that section, so that a default watchlist expiry time can be set.This default expiry only applies to (a) pages that are currently unwatched, and (b) pages whose watchlist expiry will expire before the default period I set in my preferences (e.g. if the page will leave my watchlist in 2 days, but my preferences stipulate a default expiry of 7 days, the page's watch expiry will be extended to 7 days).
Who would benefit: Wikignomes (those who mainly perform maintenance edits across a wide variety of pages)
More comments: The amount of development work involved is low. It probably would take an experienced MediaWiki developer 1 or 2 days (not including any design required).