Mainly for beginners
Here the rear wheel is constructed with ratchet and pawl so that it can't go backward. Your foot can't slip out to the rear (as is common with little kids learning to skate).
In fact, unlike ice skates, people rarely skate backward on roller skates. It's too dangerous. So, it is likely that this feature would be kept to give a firmer "push off" without needing to angle the skate. It should however be made easy to remove the pawl to disable the function, i.e. remove cover and screw holding pawl.
The second feature needed is a temporary front brake
Here, a bolt and nut can be tightened on the front wheel axle to provide friction thus preventing the front from slipping out too much. This is strictly a "newbie" device and would be loosened gradually to allow more free motion. Every few minutes of practice would require the loosening of the nut as the new skater learned to stay upright. I believe this feature would be completely and permanently disabled after the first few days of learning.
I tried to devise an adjustable, centrifugal front brake that could remain on the skate at all times but it would be too difficult and expensive to manufacturer and would probably end up being an electronically controlled hydraulic device. Too much engineering for such a simple thing as skating.
The above is cheap and do-able. So, when will I see it?