I was actually the lead engineer on an Openwrt router. I hadn't heard of it before that, but at one point I pretty much knew it inside and out. It's been a few years since I left that company, so I'm a bit rusty at this point.
We made tons of custom features for our router. I did the backend and implemented UIs for most of them. The biggest feature I did though was a full REST API to be able to configure the router from a smart home controller, which was the company's main product. I did both the router side (server) and the smart home controller side (client/caller), including the UI on the smart home controller. I spent almost a year on just that feature. But I was damn proud of it by the end.