Keeping in mind that aside from the Star Wars pack, I'm basically always wrong...
I'm hoping for a Monte Carlo casino pack with some romance and elders content and maybe, if there's room enough, stage performance in it.
My reasoning? There's been a lot of eloping content in terms of Sims Spark'd/the patch content. Plus in that eloping challenge, the casino location was the only location we don't actively have in our game, which implies to me that we'll get it soon. Plus elders and casinos often go hand-in-hand and there was an elders competition in the Sims Spark'd as well. The pink plumbob that a simguru posted could be in relation to something Showtime, as some suggested, though I don't know if a hint related to a previous sims game makes sense, so I assumed it was more about the flirty mood, suggesting romance. And I think a romance addition with a casino pack would be great. And to be fair, shows are a big part of casinos, so it would make sense to squeeze that in too, but that could be too much content for one expansion pack.
Though really, if the big feature was a performance system, and then a smaller gambling system tied to money management and objects and a new lot type, plus a small system for romance interactions, maybe it'd be perfectly fine. Consider Get Famous, with its elaborate acting system, plus the fame system, plus the patched-in reputation system. Plus a ton of acting-specific objects. I'd say it's doable!
Like I said, though, I'm usually wrong. This is also very much something I want, so I'm also mostly hoping it's this.
But if, by some miracle, it's casinos, I called it this one time.