I'd be real interested to know which path GlassWire are going to take. But I'd be very surprised it the GlassWire team got into a custom router OS or proxy server because there are so many good products that already monitor networks.
GlassWire's strength is in making it easy to see what each process is doing on the network. So they'd be far better off working with existing standards (e.g. SNMP) and integrating with those network firewalls and the like.
I think that they have other options than passive capturing packets on the network capture and port mirroring at the switch. For example, Cisco Netflow, unlike sampling protocols like sFlow, provides sufficient detail for much of the traffic monitoring (incoming interface, IP protocol, source and destination IP addresses and ports, etc).
The major imponderable, from my point of view, would be how many device types will need to run GlassWire when those devices don't provide an interface to get enough (or any) process info. In other words, I don't think that they'll have their software on every home appliance connected to the network unless a) it uses a major OS and b) the device is resourced (CPU, RAM, etc) to cope with the extra load.