Regarding XQuery we just added JSON querying on top in Brackit[1] / SirixDB[2].
Brackit is a retargetable query compiler and does a lot of optimizations at compile time as for instance optimizing joins and aggregations. It is useable as an in-memory processor or as a query processor of a database system.
The Ph.D. thesis of Sebastian:
Separating Key Concerns in Query Processing - Set Orientation, Physical Data Independence, and Parallelism
http://wwwlgis.informatik.uni-kl.de/cms/fileadmin/publicatio...
[2] https://sirix.io