Quests are not plugins. It's not like a Skyrim mod where you just download an existing quest, plug it into your game, and suddenly new content exists.
You have to make the quest yourself.
I don't know what kind of plugins you are looking for as there are literally countless numbers of plugins available here, on itch, git, and on personal websites by developers from all across the world.
It might be a good idea to plan your game a little bit and see what you need, and then put in requests for specific types of plugins to help you achieve what you want.
Broadly speaking, I suppose whatever VS's free pack offers is a good starting place.