Scripting support would be cool. To me, Rust isn't necessarily the best language to write small one-off scripts in; I personally prefer dynamically typed languages (or whole-program type-inferred languages) for that. But if you want to write scripts in Rust, I'd certainly review any pull requests to make it easier :)
I'm interested in this from a vision point of view. What are going to be your first "written-using-Rust" apps ? Firefox is gargantuan, so it cant be your first. Ergo, the question about platform toolkit compatibility. If you build that one first, you will start spawning an ecosystem almost instantly... if you want.
Mono, Vala, etc. have nice apps that people love. I am wondering on how can you bridge the gap between javascript/Qtscript and Rust to make it easier to build apps in.
Rust compiler is written in rust, so that would be the first "written-using-Rust" app. Additionally afaik the work on rust-based web-browser (engine) has began, so that would be second (major) project for rust.