So every station needs to be able to retail every ticket type and accept every payment type?
That would be the obvious requirement. However I guess the counter argument will run similar to that being used against cash payers at present: If you don't like what the TVM offers, you can always buy in advance, on your mobile, get someone to do it for you etc.
Compulsory ticketing, without compulsory
suitable ticket buying opportunities, is another obstacle to deter people from travelling. An idea placing burdens upon others without placing burdens upon those proposing the idea. There are instances, raised many times, where there is only a single ticket machine and crossing the tracks to buy a ticket and returning to the other platform can be a time consuming exercise. So a buying opportunity at every entrance and/or on every platform would be one of my requirements for the proposal to be acceptable.
There is the usual distinction, discussed recently, between long distance (more likely to know their travel in advance) and short distance travellers (more spur of the moment) using the same service. Similarly there is a distinction between lines where there is a turn-up-and-go frequency rather than an infrequent service.
Having had two TVM fails recently, neither my fault (EMR, Uttoxeter, a) circling 'in progress' thing yet nothing ever moved on or reset, and b) card declined as PIN required but it didn't actually say that on screen) I am not keen on the idea myself, but then I always carry cash to 'buy' myself out of any such problem by paying the guard.
There might be a lot fewer types of ticket in the future if the London model is implemented elsewhere.
At some point it seems fairly certain that London itself will go Oyster / Contactless only.
Outside of major urban areas, I guess not, as the latest 'ticket simplification' seems to have stalled and I dread to think of the contactless/capping arguments which would ensue for long distance travel. Fancy having a few hundred quid headroom in your account at all times just in case Avanti charge you the max from London to Manchester for reasons only known to the software alogorithms?