In my opinion it's quite obvious that it will be 3D was not only is it called construct 3 (it would be misleading otherwise) but because there isn't enough for them to improve so that they can make an entirely new engine. Furthermore there aren't any 3D drag and drop engines as they all use C which is hard to learn, this means that a 3D drag and drop engine would be very popular as many people want to recreate the games they play (which are almost all 3D)in there own style.
Bottom line- the name would be misleading and the engine would open up a whole new audience.