You may have heard of it already, but if you haven't, I would recommend taking a look at RethinkDb: https://www.rethinkdb.com/docs/architecture/

It is a NoSQL Db, but has a lot of the features you're talking about. It doesn't have any form of "schema", but does allow for SQL style relational data. It definitely supports your "dynamic relational" data idea as stated above. The only issue is that any sort of data constraints need to be addressed at the outside of the Db itself.