FoundationDB have announced a their ‘Document Layer’ and it’s compatible with MongoDb’s API - https://www.foundationdb.org/blog/announcing-document-layer/
There are plenty of things to be excited about here. See this list for advantages of FDB over Mongo. Also, I believe since the Document Layer is just a stateless layer, the data behind it could could be queried in other ways e.g. as keys and values, giving us possibilities that Mongo doesn’t.
We should investigate to see what advantages like better performance FDB could provide.
Currently, FDB doesn’t have an Oplog since it doesn’t need it itself, however they are talking about adding one.
It would be especially great if it could provide us with this ability: https://jira.mongodb.org/browse/SERVER-31720 - “get a notification when a document stops being matched by a query”. That could give meteor apps a huge performance boost.