The cross-wiki watchlist proposal is likely to be in the Wishlist Survey top 10. There's an existing ticket -- T5525: Cross-wiki watchlists -- and there may be other discussions about this long-standing community wish.
This card is preliminary investigation -- see what ideas have been proposed and rejected, which seem promising.
Also look at the existing 3rd party tool: https://tools.wmflabs.org/crosswatch/ -- especially how it handles filtering.