All Public Call 2018 prompts/requests are public on AO3!
You can write treats (additional stories) for the participants whether or not you're signed up for the challenge. Just pick a prompt, write, and upload your story to the Public Call 2018 collection, gifting it to the recipient in question. (There's a quick how-to under the cut at the end of this post.)
Treats will be revealed on December 21st (after all regular gifts have been revealed). You have until then to write and post treats. Have fun!
How to post a treat: Go to the collection page and click the "post to collection" button (upper right, under the collection title). Under "Gift this work to", fill in your recipient's name. That's all!