Ask HN: Which are good pet projects to get into Back End Programming?

I am a Front End developer for several years now, and did also a fair amount of Java (and even PL/I) in the past. When I start side projects, I always focus on Web Applications. For this, I mostly develop some basic NodeJS REST application for the backend.

This being said, I am lacking some nice ideas or projects to get into the back end. I would like to try Go, Rust or whatever, just to get a new experience and insights.

So, Back End people at HN, which are nice ideas to to develop over the course of a few days? Or where can I read more about nice project ideas?