the politics of the idea are very complex. The Oyster extension agreements beyond the Greater London Authority boundary require agreements with the local councils to ensure they have some level of oversight- as the locals there have no electoral influence over the GLA or Mayor of London. Further expansion will require many more agreements to be made.
part of the trouble is that some services that might be regarded as "Suburban" continue a very long way into the surrounding country- for example, trains from Moorgate run out as far as Letchworth. It would be a much simpler affair if, on every line, there was an "interchange" station that the TfL services could terminate and other commuter trains could run fast between it and their terminus- ideally, the "metro" would run along seperate tracks, but bear in mind the "fast" lines are often already full with express and intercity trains, so the slower commuter trains can't just be shunted out there.
Plus, those stations just don't exist.
Politically perhaps some of these issues could be circumvented by expanding "London"- adding a further ring perhaps of boroughs that currently lie outside the Greater London boundary to the sphere of influence of the Mayor. This would be complex however.
