In a $setWindowFields stage, if one of the 'output' fields ends up not being used, we could rewrite the query to avoid computing that field:

{$setWindowFields: {
    output: {
        total: {$sum: "$x"},
        avg: {$avg: "$x"},
    }
}},
{$unset: 'avg'}

would optimize to:

{$setWindowFields: {
    output: {
        total: {$sum: "$x"},
    }
}},
{$unset: 'avg'}

It might make sense to do this for unused $project fields first: SERVER-55886