In terms of IOA having low attendance- i think that is just because it is new. What i was more refering to is that the people who DO actually go there really like it. Although- i can see what you mean about attendance, the park seems litterally empty everytime i go and they make such huge queues that never get even 1/2 full.
But back to the original question, if i was in charge, i wouldnt open up a 5th park for a while. Instead, I too would try and improve the currently exsisting ones. I think what they're doing with Epcot is great, adding interesting, thrilling , exciting rides but keeping them educational in the same way. As for the Magic Kingdom, i think they should have kepy AE and put Stitch somewhere else. Also, tear down Toontown Fair and put in a decent toontown with a Roger Rabbits Cartton Spin. Then, update Fantasyland by adding new attractions like Alice in Wonderland and Pinacchio (i'm not sure if thats spelt right) or even some new ones like...Bambi or Robin Hood- i dont know but some new dark rides would be nice, especially to take away the empty lagoon. Then, to get into the "thrills" category, the MK should put in an updated Materhorn similar to DLP's Space Mountain. Finally, add an Indiana Jones ride to Adventureland- but not a clone, a completly new ride. This would ensure the MK have thrills, and tamer rides. IE: something for everyone. Thats what makes IOA so popular. They have so many different types of rides- all of which are excelently done (ex: Cat in the Hat versus cardboard cutouts on Disney dark rides). And then, instead of adding a Disney Sea, why not make a new land at the AK with a sea theme? It could have sea animals, a few rides, maybe an Aquatopia......
Anyways, those are my ideas