Every time I'm going for airfilming I concern about the worse senario in this job: The main controller fails and my $5000+ investment falls uncontrolled and with high speed in to earth. And all that happening in front of my customers eyes! Well....I had an idea. What about designing a smaller some called "buffer" unit that senses the presence of the main controller and in case he fails, its purpose is returning our model safely back to home showing us in our FPV screen the cause of the issue (a message like "main controller absence").

In some sense, the storage of the home point and the ability to return to it is one step in this direction.
I think all software developers would love to have a "perfect" state but in flight, I can't imagine what that data would look like, especially with me on the sticks! Your idea is a good one, if you could have a perfect state to compare to in the milisecond before failure.

It's easy for DJI to implement that. The algorithms can be the same as in the main controller without the tx command sections, and this unit does not have to have the signal reading circuitry from receiver outputs. Only the GPS antenna input is needed, the inputs to connect to servo outputs of the main controller, an input to sense the presence of the main controller and of course the outputs to speed controllers. It will act as an intermediate from main controller to speed controllers.