That's a fun question. I'm not entirely sure. The only feature request I would have is maybe a few more hints or links to documentation. I'm learning Rust and tackling the Redis challenge, and I got stuck on one point because I don't know enough about how to parse strings in Rust. In Go or JS it would be very easy for me, but since I don't know Rust, I'm spending a ton of time poking around in some Rust tutorials and docs to find what I need.
But back to your original question, some random things that might be fun:
