I feel like it will be something small and everybody would be disappointed after all the speculations (but that's a must for Sims4 hype strategy).
And here's a crazy idea - this won't be Spa or Outdoor Retreat... It will be ROM instead and they'll update Scruberoo with a couple of missing packs integration options. Like cleaning Bust the Dust & Laundry with it. At least this will explain this "clean, clean" tweet. That's the size of update I'm expecting.
From the other hand if speculations/expectations are right and this is something on a bigger scale and for SPA I hope for a couple of new traits and Wellness aspiration. Both traits and aspirations are seriously lucking in this game so adding them with older pack would be a good idea probably.