The Presto SQL Parser was perfect for phase one of the SQLHandler. It was nicely split off from the larger Presto project and it did everything that was needed for the initial implementation.
Phase two of the SQL work though will require an optimizer. Here is where Apache Calcite comes into play. It has a battle tested cost based optimizer and has been integrated into Apache Drill and Hive.
This work can begin in trunk following the 6.0 release. The final query plans will continue to be translated to Streaming API objects (TupleStreams), so continued work on the JDBC driver should plug in nicely with the Calcite work.