Aggregation pipeline support preview will allow Azure Cosmos DB developers using MongoDB API to perform data manipulation in multistage pipelines even within a single query, enabling the streamlined development of more sophisticated applications. Aggregation pipeline can be turned on in Azure portal. Unique index capability is now generally available and will allow you to introduce the uniqueness constraint on any document fields that are already auto-indexed in Azure Cosmos DB. Azure Cosmos DB now implements MongoDB 3.4 wire protocol, allowing the use of tools and applications that rely on it.
Learn more about aggregation pipeline support preview.
Source: Azure Roadmap