Actually they've likely already begun work on the expansions long before the game released. This is what they did with Witcher 3. Hearts of Stone was a little over 10 hours long and added a ton of content, while Blood and Wine was almost the size of a new game with a massive new area and over 20 hours of new story not counting the side quests. Both of these expansions were released within the year of Witcher 3's release, with Blood and Wine releasing pretty much 1 year after the game came out. With the size of those expansions, there was no way they developed that after Witcher 3's release. Especially when you take into account the Night To Remember trailer that came out before Witcher 3 launched, those events didn't take place until the final expansion.
We were supposed to receive a roadmap of the expansions by launch, but this was delayed when the game was delayed to Dec 10. Hopefully they'll announce it when we get the DLC drip early this year. It should give us an idea of what to expect in the expansions. Witcher 3 announced theirs prior/at launch (I don't recall which) and gave brief descriptions for them. That said, we didn't know Blood and Wine took place post ending until later.
Honestly the best thing you can do is continue to make your voices heard. As someone mentioned before, CDPR added more romance content to the vanilla game through a patch for Triss after an outpouring of fan feedback. There's no reason to take the foot off the pedal now. Keep letting them know what you want to see.