Well, I can think of two basic themes/ideas for a 3rd gate (WonderSea Island is separate)
1. Universal Classics (Maybe a showcase of film history - not too different from World Showcase or the period areas of DCA) - Not a great idea
2. Discovery/Voyage (DisneySea for Universal)
It could have:
1. Transylvania (Van Helsing E-ticket using Pandora's Box, Frankenstein walkthrough, Black Lagoon submarine ride) with Horror Make-Up Show, Beetlejuice and Monsters Cafe moving here
2. Skull Island (Kong 360, Skull Island flume, etc)
3. Amity (new Jaws ride, stunt show, mini-carnival, etc)
4. a new version of Lost Continent (the old one can be split - 65% goes to Seuss, 35% goes to Potter) themed to just Greek mythology with a Poseidon E-ticket, a new Sinbad ride, etc
5. Middle-Earth (the main area of the new park) with a trackless E-ticket boat ride, complete immersion, and a top-notch show-stopping TOT-style ride themed to Sauron's tower
6. Hyrule (themed to Legend of Zelda) - Family-friendly dark ride, Splash Battle ride, state-of-the-art coaster in Zelda's Castle, amazing show
7. Mushroom Kingdom (adds family-friendly appeal) - Mario Kart ride similar to Autopia, E-ticket dark ride, a new coaster system ("bouncing" system of some sort) for a Yoshi coaster
8. Prehistoric area (Flintstones, Croods, etc) or DreamWorks area (HTTYD, Shrek, The Croods, Madagascar, and Kung Fu Panda) - either one
It might be too similar to IOA, but it would be an area where you can "discover" new realms (each one could be a different time period)
I think the second one has more potential than the first one (it would just become a dumping ground for old rides lol)
Either way, I love that USO is expanding like it's no one's business... can't wait until 2020 when USF and IOA leave DHS, and AK in the dust while a small Star Wars miniland, a retheme of Imagination! to Phineas and Ferb, and a Avatar edition of Soarin' is all Disney uses to combat the onslaught haha
Hoping Disney will wake up and see that Universal and even Busch are becoming legitimate competitors