None of the above. I'm not being gloom and doom, only I don't think it's going to be that big. (And some of these ideas are off the table, anyway, at least for the near future.) I was going for spiral stairs, but that isn't something you could spend much time playing.
Maybe the attaction system, since you could spend a lot of time editing your existing Sims, and you could also call it a tool. However, you'd have to assume that game changers had loads of screenshots of the menu or method for editing attraction, since the mysterious something had to be removed from many of their videos. (Personally, I think an attraction system would be boring, but I almost never play couples.)