Congratulations!!! if you have made it so far, you now have a grasp of how to implement a Lisp interpreter in Rust. A few easy things to try next would be to add support for float point numbers, unsigned integers, and strings. Once you are able to successfully make these changes the next thing to try would be to add support for the List datatype followed by support for the functional constructs of map, filter, and reduce.
With this knowledge, you should be then able to implement an interpreter for any non-trivial language.