I'd like to see is a new configuration option in the Web UI for the DHCP server. This option would allow you to specify one or more additional IP addresses of other pi-holes for alternate DNS servers. The pi-hole would always include it's own address in the list, but if you set up a second pi-hole, the one that is the DHCP server could send out the IP address for both pi-holes.
This has had some previous discussion, e.g. Secondary DNS Server for DHCP & Can the Pi-Hole DHCP send out a second DNS server address to clients?
I think of this as a small first step towards High availability (HA) for Pi-hole (running two Pi-hole's)