What you should build depends on what you want to learn. I built a full-stack app to learn server-client, a B2B application forced me to learn DevOps. Now I'm back to algorithms and Go by working on https://cuelang.org
What would you like to learn?
I work mostly in server-side apps with Python and I thought trying Go is a good idea.
You could build a server, but I'd personally find something like https://nats.io (messaging) https://fission.io (serverless) or https://redis.io (cache) more interesting. You might also look at the Kubernetes ecosystem. Building a tool or service that can interact with the API can lead to new opportunities (people hiring specifically for operator experience). I work on a code generation tool (https://github.com/hofstadter-io/hof) and https://github.com/cuelang/cue
Go's source is also pretty interesting and full of great things (https://github.com/golang/go). Here are some of the internals pulled out: https://github.com/rogpeppe/go-internal
In my case it was a synthesizer. "How do I make sound from numbers?" sent me down a rabbit hole that forced me to grapple with DSP math, bit fucking, and manual memory management: stuff you never encounter doing the usual CRUD nonsense.
And most importantly, it was fun. If your project feels like work, drop it. I will never waste my free time messing with webdev, architecture, testing, etc because those things are boring and suck and I'm not being paid to care.
