I can see there being enough in a Romance Pack for a Game Pack (with CAS/BB and maybe a 5 lot world), but not an expansion pack. I mean, I could be wrong, but I just don't see romance systems, no matter how robust, as being a "big" enough thing to warrant it's own expansion pack based on what's been included in packs to date. I think it's too narrow a theme for an expansion pack, when compared to the existing game and expansion packs for Sims 4. I do think that even with Moschino and Paranormal Stuff as examples, it's too big for a Stuff pack, but big enough for an Expansion pack? I'm doubtful.
Yes, I have my prejudices here as a pack focused just on Romance has basically no appeal to me at all, though unlike some ideas that don't appeal, I don't think a romance pack would actively impede or otherwise disrupt my current gameplay. I'd just be more interested if it had something else as well ... and getting that something else is more likely with an Expansion pack than a game pack.
But then again, if it does come in an expansion pack, I can always wait for a sale. :)