SpecialWatchlist should extend SpecialRecentChanges. This has already been done for SpecialRecentchangeslinked back in 2008 in r36682, it should be pretty straightforward (but extremely boring) to do it for Watchlist, too.
Another commit to look at for inspiration might be r36376 (SpecialRecentChanges refactoring).
Version: 1.22.0
Severity: enhancement