Wow, this is the quality of responses you only get on Medium :)
Actually, I agree that the good way to start is to take a vertical approach.
But messaging, in my view, is only getting started and there are interesting avenues to explore with regard to content discovery using messaging bots (on platforms like the Telegram, Slack).
So the blockchain becomes the persistent identity and messaging layer for bots. Instead of relying on the centralised discovery models for bots on platforms like FB, Slack, Whatsapp or Telegram you could have independent bot identities on the blockchain.
These identities could send time-stamped messages between themselves effectively creating a bot-graph. But that’s probably a topic for a longer discussion :)