Scott,
That would be outstanding!! I'm in for the 20th Century and the Daylight.. and if you were to do an El Cap in 18" I'm all over it.
The 21" cars are beautiful and no one does them better than you but they are just too big for my layout and most of the average sized layouts out there. Lionel has pretty much abandoned the 18" market and the few sets made by MTH are all plastic. I believe there is a large market for that size car especially if made from ALUMINUM!
GO FOR IT!!