Once each of us set up a system with tags and other metadata, it becomes essential to enforce it or have a central place showing us what is missing.
for example:
We may set that state tag can be wip or done.
So when you start typing #state/ there will be autocomplete for these 2 options, and if anything else is in there we will be shown an error.
This would also help prevent misspelling the word which oftentimes end in multiple variations of the same concept.
Tags do autocomplete after the first use. So creating a new tag is simply a matter of not choosing an available tag. If a tag is already in use in your vault, the autocomplete suggester kicks in as soon as you start to type.
If you have tags that are misspelled or you want to make mass changes to tag names the Tag Wrangler plugin from PJ Eby is your friend. 
@laughingmaus you are 100% right.
I tried to simplify the plugin request and pointed to something irrelevant on the way 
What this can be useful for is metadata that is not linked.
For example, dataview special tags like something:: value.
It is used in dataview, but on many other plugins like “tracker”.
Good point. I am not using dataview or tracker right now, in part because whenever I automate, I forget. I need a little bit of hands-on to keep me moving.
I’m sure there are plenty of other users who would love it though if those special tags had autocomplete.