I really wish Google's Voice/Hangouts systems were more fleshed out and full featured. Where Voice takes care of SMS but not MMS, Hangouts manages messaging with too many features. If Google, Microsoft, and Apple could simply include a default inter-operative messaging protocol that wasn't SMS or MMS, we would finally be moving forward on the messaging front. Unfortunately, I don't see this happening any time soon. I'll keep my fingers crossed, though.
Expect Google Voice/Hangouts to become a full-fledged, fully functional aspect of Google Fi. Google seems to be more interested in direct revenue these days, rather than B2B... and now that they're a wireless provider with their own hardware, that's the kind of magic they can do. Apple makes iMessage work through a bunch of ad-hoc haxies on a half-dozen protocols. That's how they roll. It makes it really hard to interface with. Google, on the other hand, is going to want their ecosystem to behave for eleventy dozen vendors. They'll come up with a hard protocol. The question is whether they'll get enough marketshare that Apple users will be annoyed by their iMessages not working outside of the Apple ecosystem. There are grumblings now but we're a ways from a tipping point.