It's notoriously difficult right now for a regular weakauras user to manipulate the tooltip (see https://wowpedia.fandom.com/wiki/UIOBJECT_GameTooltip), so I think it would be a great feature if users of weakauras can just add lines to it.

I don't think so.

Why? You can already manipulate the audio of the game or send private messages. Why is this too different?

It doesn't fit as well as those things.

I know is a bit further than those in terms of simplicity but it's at least subjective. Doing stuff on the chat mechanics is also a bit obscure for most users; I do accept the tooltip will be a more niche use so if you say 'it's too niche' I get it.
But theoretically it can expand to include things even more "niche" than the tooltip because its main job appears to be to offer to users who aren't good at addon-authoring a short-cut way to manipulate the game based on triggers.