Hi, Pi-hole so far so good. I would like to request Pi-hole developers to integrate the following features:
Hoping this request is not huge.
Marc.
But seriously:
This already exists (e.g.):
For a LAN-LAN-firewall realised with a Raspberry Pi you need a second LAN interface (via USB), only RasPi 3B+ an 4B provide Gbit-Ethernet (1000Mbit/s) and only a RasPi 4B is equipped with USB 3.0 for Gbit USB-LAN. The RasPi firewall will be the bottleneck in your network!
Maybe other Hardware suits you more:
Deny or allow access to specific domains on demand by using a script like:
#!/bin/bash
pihole whitelist domain1.tld domain2.tld
to grant access and
#!/bin/bash
pihole whitelist -d domain1.tld domain2.tld
or
#!/bin/bash
pihole blacklist domain1.tld domain2.tld
to block access!
From the Pi-hole documentation: Editing Whitelist and Blacklist
For more details on scripting: Bash Scripting Tutorial
And to automate the execution of the scripts use cron:
A Beginners Guide To Cron Jobs
Addendum: similar request here: Second level Blacklist triggered on a schedule
So you really want Pi-hole to cook you coffee?! 
Yes I am serious! You thought I was joking? Have you seen Amazon EERO? It has all the above and more. Pi-hole replaces many paid services and hardware alternatives available in the market and am hoping see one day it does what EERO does.
Hope you did not get offended by my request.
Regards, Marc.
So many people confuse router and access point. Very different and separate beast. A router can contain an access point but a router is not an access point. Gotta love the consumer crap out there that has basically confused the public. On a side note a Raspberry Pi could not handle routing any decent connection. Not only do you need a minimum of two ethernet ports, the single onboard port it does have can't even handle the speed it's rated for. On a side note you could technically route with a single port using vpn concept but it's way above the skill set of most users. We're also back to the under powered hardware which would seriously choke the network.
I'm interested in the idea of Parental Controls, but my philosophy is quite a bit different. Instead of policing where kids can go (with whitelists and blacklists), I'm interested in ways of limiting the kids daily internet usage to, say, 2 hours of online time or less, regardless of which computing device they use, within a household.
I'd like to see the Pi-hole have the ability to do a captive portal-type asking the kid for three initials (which acts as a semi-anonymized username, no password needed), then log the amount of time for that 3-initial person in a simple database, like say, sqlite. Then the captive portal could jump in after the total time has run out. It would be an honor system that the kid uses the same 3 initials each time. The parents can see if other usernames got created by kids who try to cheat the system. Stern language and sanctions on desserts, and other such goodies would be the policing from there.
Maybe a custom browser Add-on (which talks to the Pi-Hole on it's own separate port) could give the kid a running count-down timer, so they know how much time remains in their 2-hour-per day budget. Parents should be able to specify whatever length time they'd like to allow in a day, but 2 hours is the default. Parents could bypass the limit themselves, because they enter an admin password (just for this feature) into the captive portal, disabling it for that device (or user profile in their browser, more specifically).
The Alexa and Google integration doesn't interest me at all.
The Raspberry Pi 4B has a true GbE port, which can go like 110MB/sec. Inexpensive, good quality, Linux-compatible GbE USB 3.0 dongles exist, such as this UGreen one. I have one of those personally, and it goes full GbE speeds (I've tested it in Linux with iperf3).
These are on separate buses: the onboard GbE, and the USB 3.0. If people select their hardware carefully, I think it might make a pretty good router. But I doubt the onboard wifi can handle all that much simultaneous traffic (for an entire LAN worth of wifi devices, all connecting to the RPi 4 as the gateway). Ouch, don't expect more than about 10 MB/sec, tops, from an RPi 4, over wifi!
(Here is a chart from there:)
If most traffic on the LAN was ethernet, not wifi, then maybe. If the RPi 4 shut off its wifi, and let some other home router device do the wifi access point (but not gateway role, handing all that traffic over to the RPi 4 over GbE), that might be a nice combo.