The A330-300 is said to aircraft with the lowest seat/mile costs in its class. Although it has been selling well, the excellent 777-200ER with similar seat capacity but extended range steals away orders from many airlines that want to fly pacific routes and Europe - Far East routes.
The A340-500 can do this job as can the -600, but both are considerably larger and have 4 engines. They are directly competing with the new B777-300ER. Many airlines don´t need /want the 4 engines for all their routes. That´s why the B777-200ER is doing so well.
Specification could match but not exceed the B777-200ER capacity/range ; 300 passengers (3 class), range 14000km, about 4000 km more then the current A330-300 versions.
Modification would include increased fuel, MTOW, landing gear modifications (~A340-500) and significantly increased engine power. Probably no versions of the existing Pratts, CF6 or Trent 700s could do the job.
Versions of the state of the art GP7200 or Trent 900 used on A380 however would exactly meet the 75-80k trust requirements.
Modification to the wing & wingpylon and possibly a little higher landing gear would be required to benefit from the 110+ inch fan / high by-pass ratio efficiency.
Result would be a state of the art twin that can do 300 passenger & 32 LD3 MTOW flights highly efficient & unrestricted from Asia to USA and Europe.
Obviously the question is what would happen to A340-300 sales. This versions sales would probably suffer. But Airbus won´t mind since all frames come from the same production lines. If its a variant that sells better, they won´t object.
Goal would be to snoop off e.g. 20-30% of the B777-200ER sales. That would easily justify the investments.
Strategically : even if Airbus takes it´s time a A330-300ER could still fly a few years before a possible Boeing 7E7 stretch variant (2010-11). Not much new technology would be involved, a bit the existing types combined.
Basically it is the same process as Boeing went through with B777-300 > B777-300ER and is planning with the B737-900 > B737-900ERX...