So what do you think they're announcing during the next Maxis Monthly?

There are definitely two separate features on the way. Even if the Positivity Challenge was the only additional feature coming in the next patch the game changers would still not be allowed to say anything about it regardless of the content leak by the gurus.

Sims Community has indirectly confirmed that the challenge is not the mystery feature because he released a video about it 4 days ago. He included screenshots from Sims Camp of the positivity posters you collect during the challenge. Since the game changers were made to delete or remove all content about the mystery feature from their capture sessions, Sims Community would not have been able to share those screenshots.

Video link for those who are curious:
https://youtu.be/E9NbQNioTUk