There may not necessarily be a difference in the types of actions available with slash commands and META keys. I guess that is mainly due to historical reasons and Slack compatibility. Some things should perhaps be slash commands, such as the one you suggest (/header).
I think the META key should be used to develop a "complete" set of keyboard shortcuts to control/handle everyday actions such as replying, pinning, editing, deleting, and so on. At least that's where the top priority should be. I also think that this leaves us more freedom to follow Slack and others on potential slash commands that are or may become available.
And, perhaps the most important reason of all, using this "system" will help us avoid running into conflicts with the web browser, regardless of OS platform and what not.
:mytwocents: 