There are a ton of magic links which expand in the comments, some of which are main, meta, or community specific, and some which are templatized (on the surface, I'm not sure if they are explicitly defined on the backend). I would like to suggest an improvement to the comment editor to list autocompletion for the tags which are relevant for each site.
There is similar functionality for autocompleting
@username if that user has participated in that specific comment thread, although it is limited to showing only a tooltip with a single username. I would propose this be a more robust list of items to complete similar to how IntelliSense or other code completion popups function.