A nationally expandable system based on charging "normal" fares (converted to singles, but I believe KeyGo can handle stuff like returns and break of journey properly), a bit more like how KeyGo works. As someone who prefers to travel on walk-up fares I'd love to be able to tap in for a trip to the Lake District (say) - why not?
It seems what we'll get is a load of separate zones, which is very much sub-optimal.
Easiest, but not best. A system based on London's concentric zones is only going to handle journeys to/from London properly. Though I suppose it'll be nice if a Marston Vale or East West Rail trip ends up being single-zone and so costing next to nothing!