You're saying you'd like SMS integration? So if you add your friend with a dumb phone to your Hangout, they can participate via SMS?

That would be awesome. I have a few friends with dumb phones that are constantly left out of iMessage conversations. I feel bad about it, but not bad enough to inconvenience myself (or pay for an SMS plan; yes I'm a terrible person).

Edit: The federation thing is probably a pipe dream. No one has an incentive for supporting a federated protocol -- at this point it's either existing platforms going for lock-in (the iOS lock-in caused by iMessages is huge; it's the biggest reason I don't consider Android). I guess what I really want is iMessages that works on Android and over SMS, and doesn't suck at syncing between devices.