As others have said, if you are talking about an all line anytime ticket then I dont see how it could work from a cost point of view. We dont have the IC - Regional segregation that Germany has.
If you are talking about an any mode, any operator area ticket then that could be very useful, but given the current state of public transport I dont see how it could be made to work.
Given the current state of ticketing in this country, and the fact that you end up having to split tickets to get the best deal on a significant number of longer journeys then to me any money and effort would be better spent getting to a much simpler but revenue similar system. The changes made recently are moving us in the opposite direction, I am thinking here the LNER single journey pricing, which is fine if your journey is LNER only, but as soon as you introduce a 2nd operator, e.g. Northern then it breaks down as without splitting your ticket end to end fares have increased significantly on some journeys.