Interview Take-Home Project Chat API

I've been asked to build a simple chat API as part of a take home project for an interview. I'm supposed to disregard common features like security, authentication, authorization, and user registration. I won't share the spec here, nor am I asking for help building it. I am curious, about ways to improve upon a super basic API design.

How would you go "above and beyond" building a chat API? What kind of features/design would impress you if you were on the other end?

Thanks for the ideas!

edit: they specifically mentioned that sneaking in "something cool" would be a bonus