Well the A320 already has 4 derivatives - 318 319 320 & 321 - i can't see anymore room for development there ! Apart from NG
The cabins are already very modern, tho could do with a redesign soon.
The 340 as mentioned has already had 2 major recent developments with the 500 & 600.
I believe the current 300 is a NG
The 330 is still very modern,2 good versions,and perhaps the natural path would be an NG
version to compete with the 7E7, though of course it is bigger.
I think their current lineup is good to say the least.......