I'd certainly take a Convair 880/990, but I don't think it's practical for CS from a popularity stand-point.  I would guess that the next plane from CS is probably going to be a 747-1/200,  personally not my favorite plane, but it has so many requests and it seems to be what people want.  I've said it before, but I would like to see something smaller and similarly old like a BAC 1-11 or Fokker 28.  Or for something more modern how about a Sukhoi Superjet, pretty cool looking little plane.