I think a new hotel resort is probably the most important expansion that I can think of right now. The reason I come to this conclusion, I here from people on this board that their resorts are mostly always filled and it makes sense because it reflects in thier room rate. Their least expensive resort is near $150 if not higher during regular season, at least when I checked. Why would a family stay at Universal when their rates are higher then Disney's. Although honestly, I think Universals resorts are superior to Disney's resort in quality. If they open another resort they will have more room for people visiting which will fill the parks up even more.
I think one of the problems that leads to the low attendence figures at IOA was because they didn't have enough resort rooms to attract guests to stay there which equals not enough guest to visit the parks. While yes there are surrounding hotels and Disney as well, guest staying on site are more likely to spend more time in your parks then guest not on site. I think with an addition of a fourth resort accompanied by better marketing, the resort will prosper better then what they have seen in the early 2000s.
Damn, can you believe it is almost 2005, we are already saying early 2000s. SCARY!!!