I agree 100%! DataFusion is beautiful and easy to extend in any direction. For the second version of my "out-of-core" graph algorithms project, for example, I implemented my own "co-partitioning" to speed up joins and achieved a performance improvement of two times! It was also easy to modify the physical plan and declare partitioning.
The extensibility is insane, you can create your own query language that compiles to logical plans.