I personally think they should adopt something brand new and not evil. I'm not saying that RCS is evil, but from what I can tell, while it has federation, Google seems to be gaining a monopoly on RCS (sorry, I don't remember what article I read that in). My pick would be something like Matrix or even XMPP to make sure that there's a definite standard that is federated by design; however, you'd have to trust your carrier/software to not dump your messages onto Google's server instead of using a different server.