Maybe they could start by making Hangouts less of a PITA to use. I'm not a huge FB fan but they nailed it with messenger. It works nearly flawlessly even if I'm on my tablet, laptop, and phone. iMessage is still awesome but when I need to talk to my Android friends I more often than not reach for messenger because of the extra feature set.
I would like to see messaging app that is as good as FB messenger with sophisticated AI capabilities that would help me make useful queries and have fulfilling responses. Plus, if they can implement data compression to reduce traffic in video and voice calls, that would be golden .