What I see around the web quite frequently, is that people want to multi-task their pi’s, and run other webservices alongside.
What I suggest is that like many applications, (ie. plex, sonarr, couchpotato, etc…), pihole should have a contained webui.
By default it would run on port 80, but would have a setting to change that port from the webui or the initial install. Having a setting locked in the config file would be nice as it would stick between upgrades. (I run lighttpd on an alternate port, but upgrading sometimes has changed it back to port 80)
I think this would solve problems for people that choose to run nginx or apache on port 80, and just reverse proxy.
Speaking of reverse proxy, it would also be nice to be able to set a base-url/root-url. that would allow people to easily reverse proxy the webui without being stuck with /admin or a bad nginx rewrite.
Another benefit of doing this would be making parts of the pihole setup not as easily “tamperable”. I think people break things on accident when they play with other webservices.