When we compute a new field from an expression (in a $set / $addFields / $project stage), and then that field is ignored or discarded later, we could avoid computing it in the first place.  When optimizing a $set stage, we could use getDependencies() on the pipeline suffix to decide which fields are actually dead code.

We could do a similar thing for $group (and $setWindowFields).  This may have an even bigger impact, since accumulators and window functions can use a lot of space.