On a tangent, @mg on Medium suggested the idea of iMessage as a platform[1]. No one responded on HN[2] when I linked it though, so I don't think it has much traction.

[1] https://medium.com/@mg/there-s-a-chat-for-that-apple-s-bigge...

[2] https://news.ycombinator.com/item?id=9765818