I think it is a stain on TFL and this country that it cannot get all the airports internationally known as London airports - so Stansted, Luton, Heathrow and Gatwick under its Oyster card system. It must seem so confusing to tourists who land at say Stansted - can buy an Oyster card there at the station but not be able to use it! It's infact redicilous - it's also so confusing that where I live though now covered by an Oyster extension it's so hard to work out what is cheaper using Oyster PAYG or buying a paper one day travel-card ticket - some times it's one other times it's the other due to the evening Oyster Peak that doesn't apply on paper tickets. This is the only country I know of that has a two tier peak and off peak price system on trains and subway (but not busses). Would it not be so much simpler if there was one price 24/7!
Any way a great start would be trying to get extensions to cover Stansted - like you did Gatwick and then possibly Luton to. Just makes so much sense as that airport is not only a bugger to get to Greater Anglia rip you off with the express train service prices when there doesn't seem to be a regular priced train unike Gatwick - it has the expensive Gatwick Express but also lots of ordinary trains running there to.