LMML 14/32, I agree totally. The 300/310 has come to a dead end just like the 707 and 727. There will be no upgrades or direct replacements.
But Airbus has a lot of ongoing projects at different stages:
A400M (the military transport)
Anyway it is absolutely viable to try to guess about the next project(s) which may come long time before the A380 gets in the air.
If we look very closely on the 318, then it looks a lot more like a prototype "320NG family plane" than just a further shortened A319. Many changes are not very visible, but still quite important modifications. Just to mention a few:
Much of the cockpit hardware will be totally new, based on newer generations of electronics. The most visible thing will be flat screens instead of CRT tubes, but that's only the surface of the "refreshments".
Many major structures will be laser welded instead of riveted as has been the norm during the last 75 years. It will save weight, reduce maintenance and extend safe fatigue life.
As soon as such things have been tested until failure, and certified on the A318, then we should expect these things - and maybe slightly improved engine subtypes - incorporated on 319/320/321"NG" planes. That may happen even before the A380 will fly.
Airbus does have a wide performance gap between the 321 and the 330. A couple of years ago they looked at a short 330 (first named 330-100, later 330-500), but it was cancelled considered too heavy and expensive. But they need a direct 757/767 competitor.
I wouldn't rule out a "322" completely. The 320 wing won't make it. That's right. But with a revised version of the Indian Airlines 320 4-wheel boogey landing gear, and a wing center plug (and other engines) it might be feasible to extend the upper end 320 family performance considerably and produce a capacity and range wise head on 757 competitor. A whole new wing cannot be ruled out, but the 757 segment market isn't all that big. And then comes the "Airbus holy cow": Total 320 family crew rating commonality.
Wing center plugs have been used before to enlarge a successful plane. DC-9 to MD-80 and 340-300 to 340-600 are what first comes into my mind.
There will be plenty of work to do in Toulouse during the next few years.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs