“Unified” networking driver

Not long ago I when I was iritated because µtorrent’s UPnP port mapping wasn’t working when Hamachi is installed, I got an idea – which probably is stupid or crazy for one reason or another, but whatever. The idea is that instead of letting applications themselves communicate via the network interfaces themselves, a “unified” one could be provided, instead they speak with “unified interface” which then decides the best route for the packages (the “nearest” or fastest one). It would also make switching from one network interface to another – if you for example want to go from wireless to wired while still keeping all your connections alive. This would also make the local ip to always be the same, seen from applications, which I believe could be a good thing.
This is (in at least some ways) similiar to using a VPN, but without some of the drawbacks (though with some others instead, I guess).
So, at the moment, this seems like a good idea to me – but it could just as well be the opposite.