Cross-expansion functions have existed since The Sims 1, when for one of the magic spells (Makin' Magic) you needed a Black Rose (SuperStar) as an ingredient. They have also given us more of a kind of item through another expansion, making it not exclusive to one pack - the hot tub, that came with Late Night and also with Generations.
Albeit, you are right about having to reclude to speculation only for now.