Graph Databases like ArangoDB, Neo4j and OrientDB are bound to become a mainstream hype in a few years now. People promise that it will give your site with a whopping 758 individual pieces of content amazing superpowers.
Sorry, but it can't - you need content to project the appearance of magic with Graph DBs. Media organisations, government bodies and other entities with large numbers of content and document can potentially to unlock access to new insight to existing content with a CMS and Graph Databases - but only with a rich native content model that Drupal lacks. Even if it is has been a Semantic Web Citizen since days of yore and Drupal 8 now exposing a REST API.
With the Internet of Things Drupal and Other CMSes can be exposed to potentially huge amounts of data from various sensors. This data is something that does not suit necessessary suit the data model WCMSes were built for. PHP 7 and HHVM will increase performance of backend code execution.
If you gather millions of databases, why not just store it to a better suited storage like a Document Database like MongoDB or a persistent key-value storage like Redis and sync it with Drupal? You could! But with tools like Structr built directly ontop of the Neo4j Graph Database do you really need Drupal at all?
For marketing purposes maybe. If you're a Drupal shop, then definitely.