Now, Google's Nikhyl Singhal is giving us the info we want on that front. The big news is that Google Voice SMS support is coming to Hangouts "early next year". Right now, only Google Voice numbers that have been activated on Sprint can use the SMS integration in Hangouts, but Google is working to expand that. Also, Singhal is once again giving the Google line that it is working on MMS support and bringing Google Voice to international users, but "this can take some time", so basically don't hold your breath on that one.
The last bit of news from Signhal is something that may annoy quite a few people. Google says that "there are a few third-party applications that provide calling and SMS services by making unauthorized use of Google Voice", and that needs to stop in order to ensure the security of Google Voice. The apps that are making "unauthorized use" are being asked to stop by May 15, 2014. Unfortunately, there is no indication of what that means exactly.
There is no official Google Voice API, so it seems like this could mean that all 3rd party apps will be disappearing, but the wording of Google's message makes it seem like there may be an official API to transition apps onto by early next year. 3rd party apps dying won't be that big a deal for iOS or Android users, although there are some cool alternatives on Android, but this will likely mean that there will no longer be any Google Voice options for Windows Phone users aside from the website.