The 3D Classics series wasn't just separating sprites from backgrounds. There was actually a lot of work involved in making the 3D Classics series and they probably were expecting higher sales at the time. Nintendo's expectations probably were not met so they decided to not bother continuing the 3D Classics line. All the tiles/artwork had to be redone. Sure the sprites were stuck on a separate plane, but you can't add depth to NES backgrounds that way. Kirby was pretty damned impressive for me because they added so much depth to those hills. Some games would fare better than others. Super Mario Bros. 3 would have some areas that wouldn't work, but for the most part that game would ROCK in 3D. Contra is an obvious one. Bionic Commando would be pretty cool. Cabal was on the NES too, wasn't it? Oh god, to have Cabal remade into 3D!! Castlevania could work in 3D, but CV3 would be better. Some games would just work better than others. Metroid would make a bad 3D Classics title in my opinion, for example.