The router I got from my ISP already has quite some stuff built in. For example, I can enable DynDNS, plug in a drive via USB providing a SMB network drive, and enable a secondary guest WiFi.
I agree that the killer feature is still missing. That could maybe be storage-management (NAS) or HTPC (Kodi). People seem to buy such devices.
On the other hand there are nice features already: VPN Server, Seedbox, Ad-blocking-proxy
I don't see the home server being the prime selling point. Everyone needs a router, so if you created a router that was dead simple to configure and use, had some cool extra features, integrated well with your home gadgets ("Alexa, turn off the WiFi", "Alexa, setup a guest wifi network for the next 8 hours") and on top of this acted like a home server, people would buy it.
At least from a consumer product perspective.