Well since you asked, this idea has been banging in my head since I started be Mr. Teacher for my kids who're home from public school. I'll resist the urge to describe all the background a-ha moments, experiences, observations, etc. I'll just cut to the minimum viable product that's rolling around my head.
The system is a database of "topics". A topic is a very bite sized element of training. If you're teaching kindergartners math, it'd be something like "adding 0 to a number just gives back the same number". It's almost just a sentence's worth of definition.
Each topic has a handful of prerequisites. This is not some kind of taxonomy a la the Dewey Decimal System. The prerequisites are just a rough curriculum outline. For example you can't learn about adding zero to a number, unless you have already learned single digit counting up to 10 or 20. I am not an educator so exactly what is a prerequisite for what is not my forte. And this doesn't have to be super complicated. Just a nice smooth progression from simpler lower level concepts, up through more complicated higher level concepts.
Each topic is accompanied by videos. These videos are very short, because the topics are very short. Think 5min or less. For K and 1st grade they might even be just 1min or 2min. Each topic has many of them. They could be crowd sourced or produced like something you'd get from an education company, whatever. The point is: there is a back log of videos that teach each topic. So if you're topic is "adding 0 to a number just gives that same number" there would be at least 1 but maybe multiple videos in the system that teach that topic.
Each topic has a series of questions, each question has correct answers, and incorrect answers. As much as humanly possible we have a catalog of multiple choice questions for each topic. The questions can be written in a variety of ways, the correct and incorrect answers can be written in a variety of ways. For example one question might have a few ways it can be written such as "1+0=?" or "one plus zero is what?" or "one + zero" etc. The point is you can randomly select a series of questions, and you can randomly select one of the correct answers, and a few of the incorrect answers, or construct "all of the above" or "none of the above" and even if two people have the same questions in the same order, their experience and what they see might be totally different.
Finally, each student account has a list of the topics they have "mastered". So your account is like a tree of the topics you've mastered, which adjacent or peer topics you haven't mastered yet, and which topics you have mastered all the prerequisites for. I could randomly select a topic, and by walking the tree of prerequisites a little bit, find a topic quickly that is appropriate for you.
Okay those are all the nouns. Now for the verbs. How does this work? Basically the experience is a loop. A student comes in to the system. The tool randomly selects a topic that the student has mastered all the prerequisites for. They are presented with one of the videos for that topic randomly. Then they're presented with a short quiz randomly built from the questions for that topic. If they get a 100% correct on all the questions, they've mastered that topic. Wash rinse repeat until they've graduated from college.
If you get a quiz wrong, all the prerequisites for that topic are no longer mastered. In essence we "back up" one node in the prerequisite tree. Over time a student will repeat some topics more than once, as they push the outer boundary of the leaf nodes of the topic tree they've mastered. This is to be expected.
Since we are randomly selecting topics, randomly selecting videos on those topics, randomly building quizzes, and each topic is only a few minutes long, we should be able to keep this up for a hour without getting too bogged down in any one topic or any one quiz or any one subject. It should flow smoothly, quickly, without friction, without complication. I'd estimate my 1st grader could plow through 3 or 4 topics in an hour. My 4th grader should be able to easily handle 5 or 6 no problem.
The system has hardly any user interface at all. There might be a single button to say "I'm ready for the next topic". While the video plays there might be a single button to back up 30 seconds. Then the quiz begins and there is only 1 button for each of the 4 multiple choice answers, and then back to the beginning. The fidelity of the user interface is comically minimal.
The system does not have any obvious grades, percentage completion, progress reports, awards, kudos, etc. The loop of video, quiz, repeat, is all there is. If you wanted to do reporting on student's progress you could do so, outside of the system, but within it the experience is on rails. The student is not alerted to wrong answers, correct answers, whether or not they've "mastered" a topic, what the upcoming topics are, etc. None of that is presented to them.
Okay that's enough of a description. Each of these design elements is backed up by my experience being Mr. Teacher and has specific reasons for being there. I'm avoid giving all my back story because this was probably boring enough as it is. I thank the OP "plafl" because this thing has been in my head for weeks, and writing it all out has helped me get it out of my system.