Rather Wattpad specific. I'm not familiar with the tagging systems of other websites, like AO3, so this is pretty Wattpad specific.

Also, I would like to complain:

You may know that Watty has the function to block tags? The block limit is too low. If someone from Wattpad reads this, please make that limit higher. Or make no limit at all.

Now, to the actual topic.

Things you should tag:
- Fandoms, if anime and Manga, may tag multiple names (Ex: My Hero Academia/Buko No Hero Academia)
- Main character/s
- major side characters
- main ships, to be save, ship name and character X character
- side ships maybe too
- main villains (unless you don't want to spoil)
- dynamics between characters
- Alternative Universe
- if translated, Translation
- if a crossover, crossover
- themes/genre, I.e romance, mystery etc
- LGBT if it is focused
- OOC if included
- OC if included
- OC as the Main character
- anything that could require a trigger warning
- tag if it is smutty

This sounds like a lot but I am sure you rarely need to tag the whole list.

And if you need to tag the whole list, holy cow, what a fic. Side ships aren't really required to be tagged.

Depending on Fandom, it may be smart to tag a wrongly written tag to better reach readers.

I always tag Rise of the Guardians fics with a wrong written tag, because a massive part of ROTG fics on Watty write Guardian wrong.
Admittedly, this is hilarious.