Dear MongoDB team,
The possibility for kNN searches within a collection would be a real game changer. Not only in machine learning, but also in other areas you need this query method to find similar structures.
Practically, it could go in the same direction as text searches. So that you have to create an index that you can also query similarly and then sort according to the score.

I speak from my own suffering, as I had to switch from MongoDB to Open-/Elasticsearch for my current project, as they are also non-relational databases but support kNN searches. However, I like your product much more, which is why I would switch directly as soon as you also support this.

I hope this idea finds a quick implementation - Have a good day (:
Christopher K