A chat app which integrates with all major platforms. Either through their api or by having a webview with the user logged into that platform, hiding the webview and having the person chat as if it was one platform. While in discussion with a person, you can seamlessly switch between sms, email, fb, linkedin, slack etc.
Something like Franz but with those windows hidden and instead build one single interface on top. Also, instead of switching between platforms then finding the person to chat to, you chat with a person and switch between platforms.