LEGO has just teased a new set coming soon that that will be very big. The box for the LEGO Creator London Bus is 20,000 cm³ but this new set is 4.86 times bigger.
Sign are most likely pointing to the rumored updated LEGO Star Wars UCS Millennium Falcon (75192). The September 2017 LEGO Store Calendar shows two early access sets available starting September 14 so thus mystery set could be it. We should know more details soon so stay tuned.