Its already happening. its called LBO.
Local Break out is something we had to implement and means either your network only charges for local rates, or you can break out and use the local network prices of the network you are roaming on. My company have already implemented this as per EU Reg III and LBO roaming regulations. In fact, every network has done this in EU, by law.
Problem is, there are no LBO companies interested in doing this since theres no money to be made. So while we have fulfilled our end, BEREC (the governing body) screwed up and rushed this through without thinking.
We already have our own charging and call it "roam like home". It needs to be done though with 4G roaming on the increase, its easy to blow your cash quickly. Networks are moving towards roaming buckets though and its a far better solution. buy 1Gb for £5 or something like that.