I am currently using a PI 3B to monitor two separate interfaces on my network, each on a different VLAN and subnet. Currently I have to accept that all DNS requests are forwarded to one location that I choose in the setup and/or GUI. It is my belief that adding the ability to select multiple interfaces for monitoring in the GUI would be good and being able to forward a specific interface to a specific DNS provider would be even better.
In my case I was attempting to replace two Model B's with one 3B and while I'm able to monitor both networks sufficiently, I now have to choose between losing OpenDNS filtering for one network or subjecting both networks to the content filtering (something I wasn't prepared to do).
By allowing me to choose which DNS provider to send requests to based on interface, one RPi could be used to monitor two networks natively (ethernet and wireless) and potentially more if you add a USB ethernet Dongle, all the while being able to set specific upstream DNS providers to each interface.