Does it make sense to use Amazon Aurora for the metadata backing store?

The DB autoscaling (disk space) and self healing looks very attractive.

How big do these metadata dbs actually grow?

Also is my understanding that this is the critical single point of failure component (like the name node in HDFS land)?

The metadata storage is just storing metadata about what data should be loaded in Druid. If it fails, no new data is loaded until it is repaired, but the metadata store is not involved in the query path.

Also, yes, an extension to Aurora makes sense.