I'm thinking of a query like so:

db.foo.aggregate([{$replaceWith: "$x"}, {$match: {y: 1}}, {$project: {y: 1}}])

SERVER-72037 describes the work to push the $match before the $replaceWith. But even if that is complete, we would seem to stop our projection/dependency analysis at the $replaceWith stage there and miss that the query only needs "x.y", not all of "x". If "x" had many sub-fields, this could be a dramatic savings.