So - one of the biggest challenges that we have with the MongoDB used by Alteryx Server is that we continually have issues with locking (where our admins have to go in and undo locks)
Additionally - the current implementation of MongoDB connectivity does not support full Kerberos authentication which means that we're on a non-compliant install (which in a large enterprise is an uncomfortable place).
Given that a very large amount of what the server does is transactional - it would make sense to have an option to use a large-scale SQL server instead of using Mongo. For large enterprise customers, there must be flexibility to allow the databases that they have large supported instances of (my strong preference would be MS SQL 2016).
MS SQL natively supports XML so all the canvasses can be stored in native format. Additionally, MS SQL allows very fast query across XML, and given the clustering and reporting capabilities in MS SQL, this would dramatically increase our ability to self-manage our infra.
Given that Alteryx is looking more and more at large Enterprise customers - a move to a large-scale clustered SQL env as the back-end would be a very positive move.
NOTE: as we consider DB options for a SQL backend - please consider your large-scale enterprise customers. For example - MS SQL or Oracle or DB2 are all much more prevalent in enterprises than databases like Postgres - so it's important to focus on the enterprise support for the DB that you choose.