All committers have signed the CLA.

My opinion, these type of things need to be integrations. What we really need to implement is the ability for outgoing integrations to have the option of whether they should show up as commands, that would enable "custom" commands. Then we need another component which allows them to provide previews which the user can select from before it sends a message, for example how discord does it:

@graywolf336
This is absolutely the right answer.
I've used both Slack and HipChat commands like /gif that just auto-select some random gif and most of my team finds it nearly useless.

Discord does a great job, and Telegram also has a great gif selection paradigm (via the @gif inline bot) if you wanted another good example of how to do it right!

Thanks again for this pull request, however I am going to be closing this due to the coming soon #6890 which already has support for slash commands to act like native commands. Also, my earlier comment still applies and we will be getting the pop up/suggestions feature as well in the future. :)