Am aware from my adventures on this thread that Pi-Hole can be manually configured to assign a different set of IPs via a second interface (in my case wlan0).
The ask here is to bring that same level of management used for the primary Interface to any secondary interface .
Eg iin a extreme case then Pi Hole could manage the DHCP for 6 connected networks (built in ethernet , wifi plus 4 usb to ethernet/wifi dongles)
Not in scope for this request would be any form of management to control routing between interfaces or any other form of networking.
My Suggestion would be for a + option that would allow you to select from other valid/currently working network interfaces (whether thats wifi or a usb dongle etc) on your Pi to bring their IP management into the DHCP console in the GUI.
If there are no valid secondary interfaces then there would be no option to add them.