We noticed some interesting behavior for query shapes with variadic expressions (e.g. $multiply, $sum, etc). Here are some contrasting examples:

db.c.aggregate([{$group: {_id: {$multiply: [1, 2, {$const: 3}]}}}]) 

is simplified to

{"$group": {"_id": {"$multiply": "?array<?number>"}}} 

while 

db.c.aggregate([{$group: {_id: {$multiply: [1, {$const: 1}, {$expr: {$sum: [1]}}]}}}]) 

is generally preserved as

{"$group": {"_id": {"$multiply": ["?number", "?number", "$myField", {"$sum": "?array<?number>"}]}}} 

It's not clear what the desirable behavior here (there's a tradeoff between a possibly high cardinality of query shapes vs losing details of the user's original query), but it seems like the behavior between these two cases should not be so different.