I haven't seen anything on this subject on this forum but there's a rumor doing the rounds that the next Vision Line offering will in fact be 21" passenger cars and they will feature interior detailing as good as if not better than that in the 3rd Rail GM Train of Tomorrow set. I had a small part in influencing Scott Mann's decisions about that set so I really sat up and took notice of this information.
Well, that and the fact that Lionel trailed in the Vol. 2 catalog the production of a set of prototypical UP excursion train cars in 2016 caught my interest. It would make sense to do those cars in a Vision Line offering not least because (a) you could expect there to be a substantial demand for them as there's no other 3rail O gauge version and (b) the present day interiors of the excursion cars are a lot easier to model than classic era cars. You can see what I mean at this UP site where all the cars and their floor plans are shown: https://www.up.com/aboutup/spe..._equipment/index.htm
Of course all this may be speculation but I thought I'd put it out there FYI.