The one thing that's holding me back on actually using Pi-Hole is the lack of flexibility. What I'd really like to see is the ability to do various things on a per-client basis.
For example, one commenter wanted a simple "reload without blocking" functionality and the response was to use a bookmarklet plus the Pi-Hole API to disable it temporarily. This works, but the problem is that it disables it temporarily for everyone and will inevitably result in "Hey, why's the ad blocker broken?" "Oh, sorry, that was me" conversations.
Likewise, I'd also like to be able to configure block lists on a per-client basis. I don't want any Facebook stuff (for example) to resolve from my devices, but my girlfriend wants to use Facebook.
Similarly, I may want different rules on different networks. For example, I may want to restrict what my IoT network can resolve differently than my regular user network. This is really just a generalization of doing things on a per-client basis.
Currently the only solution to these types of problems is to maintain multiple Pi-Hole installations. This isn't a big deal if it's just one or two, but it doesn't scale reasonably.