If you've ever wanted to run a challenge on the Archive, that's what tag sets are for. You create a tag set to hold a list of all the tags that should be available for sign-up, even if those tags haven't been used on the Archive before now, and then add the tag set to your challenge. Tags in the tag set will then automatically show up in the sign-up form.
You can add as many moderators as you want to help you manage your tag set, and they don't need to have access to the challenge settings. You can also allow the participants in your challenge to nominate tags to be added to your set. You and any co-owners and moderators can review those nominations, and approve or reject them. You can add fandom associations to new tags in your Tag Set, or you can leave them for tag wranglers to make the associations (this may take some time).
All tag sets are listed on the main tag sets page. You can browse them to get a better idea of how they work.
Some users may choose to make their tag set publicly available for others to use in their challenges. Please note that the owners of a tag set can delete the tag set or change it without warning, so before you use someone else's tag set for your challenge, please be sure that you trust them not to change it.