I love seeing these kinds of projects, but as someone who currents owns 2 pi 3s and one elgato that are all gathering dust, I hit the wall before actually starting a project because of that pesky "what would I even do with this" thought.
So I've used Pi's in various hackathon projects, and they're great for building wearable tech things that involve audio, electronics and require internet connectivity. One time, we tried to connect one up with a Leap Motion* to create a wearable device that would turn sign language into audio speech.
But even if that's not your bag, there are plenty of other "home appliance" uses for a Raspberry Pi. Other people have mentioned PiHole, which is awesome. There's also a way to set one up (in combination with an external USB drive and powered USB hub) to act as a Time Machine backup server (there's a sort of open source version of Apple's Time Capsule software floating around). Finally, if streaming services are getting you down and you feel like flying the Jolly Roger like back in the 2000's... you can set up a Plex Server (which will work with the Plex Mobile app and many Smart TV's and Smart TV devices like Chromecast) and set up a BitTorrent app with a web front-end that you can access via local WiFi. Then you just need to write some scripts to listen for completed downloads, clean up file names and put them in the right folders for the Plex Server to find them.
*Leap Motion doesn't have arm64 drivers, but we were able to connect the device to a laptop via a long Ethernet cable and forward the USB port over that cable, then send the results back to the Pi.
1: https://github.com/taligentx/dscKeybusInterface/tree/master/...