I doubt the 331 will ever see the day. When you "shrink" an a/c, it loses some of its effeciency, i.e. its empty weight becomes too important. Airbus admits that this is a problem with the 318 (it makes sense for an airline to buy 318 if it already operates some 319,320,321; otherwise, most airlines will be better off buying 717).
I think they should come up with a 310 Next Generation instead (new avionics, fbw, new engines, new wing design, etc). There is a gap in the market now for a modern, twin aisle, 180-220 pax a/c. All of that being said, of course, IMHO.
Never let the facts get in the way of a good theory.