There's a ton of things I'd be missing to start looking at this.
1. Backend agnostic: let me run on different backends like flink/spark
2. Give me off heap memory please. Let me play dangerous and use pointers directly to optimize interactions with transforms. We wrote our own GC among other things for our tensor lib due to the GC bottlenecks and copying and the like. That's personally what I kind of like about tablesaw.
A library for 1 off adhoc analysis in memory isn't a bad start though, especially since most folks don't actually have that large of problems.