NoSQL databases offer alternatives for situations where the data being processed doesn’t fit neatly into a relational database. Different types of systems have evolved to meet different needs. Some have adopted polyglot persistence, using multiple databases systems to meet specialized needs within a system. However, such a system greatly increases the complexity in programming and maintenance.
Some relational systems have added features to deal with alternate data structures. Postgres supports the JSON data type, which would allow you to replicate features of a document database, while SQL Server has supported XML for a while, and looks to be moving toward supporting JSON as well.
One solution is Multi-Model databases, which combine several database types into one system. This gives you one system to install and maintain. Plus, one query can access data from the various models, and one transaction to ensure consistency across those models.
Two system that I’ve looked at are ArangoDB and OrientDB.
Both systems support Document, Key/Value, and Graph database types in one engine. OrientDB supports SQL, while Arrango had a SQL-like query language, although for selects the language seems to resemble LINQ where the table/collection is specified first.
Both can operate as a single node or as a cluster, and have built in replication and sharding ability.
The multi-model system is an interesting idea and seems quite useful. I’ll follow up with a post with a deeper dive into one of these systems. It is interesting that neither system implements a column family model, perhaps that falls more into a analytical system that would be separate from the transacional system anyway.

10 Reasons To Consider A Multi-model Database
Data Modeling With Multi-model Databases
Datastax Believes Multi-model Databases Are The Future