I remember, ages ago, Intel prototyped a personal server, which was basically a headless PC where people would store their data on them and tote it around, before everyone decided the cloud was the answer to everything.
I can see this being something to have in households... a server that not just did GPU stuff so a game can be played on an entry level machine and streamed back in full 4k with 120 fps, but other things as well. NAS storage is a given, but having something like S3 storage with object locking to ensure data is secured in a ransomware resistant way can be useful.
Then, there are all the other doodads a NAS can provide. Plex, media streaming, backing up remote sites like one's personal WordPress site, a way to cache cloud drive providers like Dropbox or Onedrive, allowing for access without need for additional software, easy backups, maybe even some virtualization if the device has enough CPU grunt and RAM.
However, I don't know if this would be a mainstream best-seller, or something esoteric. However, before the iPhone, people considered smartphones a niche market, because people wanted something the RAZR, or even the ROKR so they didn't have to carry a phone and an iPod with them, so having a personal server might be a niche that would get popular quickly.