Give it the features of Allo, Messages and Hangouts. Also, like Allo, it needs to be able to send msgs to anyone who's number and or email is in your google contacts and ur device contacts once they are logged into a google app on their device. Turn it on by default in GMail app. When contact replies to a chat, notify his/her contacts that he/she uses chat for msgs. Hangouts was popular and only needed the additional features that Telegram & WhatsApp had. Add the features and tie it to android.