Using Mongodb, Hadoop, Tokudb, Piwik could scale to greater limits.
This ticket is an invitation to the community to comment and discuss the Nosql support of Piwik!
Reopening as there is a lot of interest from the community to build a Nosql version of Piwik!
two years ago we have discussed this topic in the past Nosql for Piwik for a GSoC proposal: https://github.com/piwik/piwik/wiki/GSoC-2013#project-gsoc1-multiple-database-support-implement-mongodb-adapter
Well, it doesn't have to be completely No SQL. You may look into ClickHouse - the column-oriented DBMS used by Yandex for its own web analytics. It even has SQL syntax, so it doesn't require a complete rewrite of Piwik.
Yes, clickhouse would be a really nice backend!
We have just contacted Piwik's team in order to check the viability to integrate SlicingDice as a backend for Piwik. Let's see how it goes.
What about Cassandra?