I can't speak to the relative merits of Julia, but I am honestly interested in anything that seeks to produce a less memory-hungry alternative to Spark.

Rust, to me, seems like a natural enough choice. It is easy to mate it to other languages, including all the major data science ones, so it would theoretically work well as the basis for a distributed compute engine that has good support for all of them as client languages. Would the same work for Julia? IIRC, it's a bytecode compiled language, which I imagine would make it difficult to link Julia libraries from other technology stacks.