Methods that accept/return BSON documents are currently annotated with Mapping[str, Any]. Ideally we could list the acceptable BSON value types to improve type safety. For example we could define the BSON value types:

BsonType = Union[str, int, ObjectId, datatime, Timestamp, Decimal128, etc..., Mapping[str, 'BsonType'], List['BsonType']]
Mapping[str, BsonType]

Even if we list the BSON types explicitly, we may still need to include "Any" so that we accept custom types handled by the TypeRegistry.