I'm sure most of us are sick of seeing those annoying popups/modals that beg us to subscribe in their newsletter. I wish someone write a web-extension to block these modals.
If you are a Go developer, you could write some reusable HTML components. For example, a set of files (mostly HTML) that helps me to pass an array of data and it renders me a grid view.
A Telegram bot that receives a Youtube video link/ID, then download it for me. You can use youtube-dl or if you are looking for more challenge you can write your own youtube downloader.
An app like tinder to find a programmer's friend based on their skills/projects. It can use GitHub API to show their skills and profile.