Not only cross platform but cross service. The real loss in messaging in general is the lack of interoperability. One thing email did get right - protocols. The messaging wold seems to have given up on this to hoard and lock in users to silo'd messaging systems - wechat, snapchat, fb messenger, etc. How powerful would it be for people to have the ability to send messages across these systems with the support of these vendors (as opposed to the Trillian like solutions of the prior decade).