I still stick by the theory that MSHI and WWOHP once HE is open are the most well-rounded areas of IOA. Neither of them need an addition unless Doom or DC get torn down/replaced. Personally, I would replace Doom with a ride that one ups Tower of Terror. DC's plot would be great for an Everest sized coaster, Quidditch field, relocated Hippogriff, and potentially a flat ride. Hippogriff's old spot can be the Great Hall coming to Universal Hollywood. However, I know neither of those options are likely
The other areas of IOA are lacking somewhat (some more than others) -
Toon Lagoon (needs a "dry" attraction or two since the island is pretty much closed during the summer) - should become Transylvania or Middle Earth
Jurassic Park (River Adventure needs some love, Discovery Center needs to be better utilized with that rumored dark ride, the TE plot needs an attraction, potential Jurassic World overlay)
Lost Continent (as the island with no ride, this truly needs some help. Either a Whoville mini-land (Grinch and Horton), or the rumored Wonka concept)
Seuss Landing (needs an E-ticket of some sort, make Seuss Trolley all-ages somehow, a yearround show, another family dark ride)
Port of Entry (nighttime show)
Marvel and Potter are the most popular areas of the park, so they don't need anything major... however, things like the Spider-Man refurb in 2012 are great! IOA needs some C and D tickets