Data Structures that are not natively implemented in rust

I’m learning Rust and looking to build a project that’s actually useful, not just another toy example.

I want to try building something that isn’t already in the standard library, kind of like what petgraph does with graphs.

Basically, I want to implement a custom data structure from scratch, and I’m open to ideas. Maybe there’s a collection type or something you wish existed in Rust but doesn’t?

Would love to hear your thoughts or suggestions.