Hey, I was also thinking about a "marketplace" or something of the sort. Where integrations are just one click (I liked that about Slack).
This will require some API back to your mattermost server for automatically creating slash commands and incoming webhooks. I'm pretty sure this is available, but we just need to make the site and those of us that can host some bot out there, we will just register.
Then it will be a matter of contacting these guys (GrowthBot) and ask them to register as well.
Maybe we should ask for this as a feature? This is not just a feature of the application, but a web site that will host such "marketplace".