Basically, Dom3 Oceania was split into Dom4 Oceania and Pelagia, the former with more of an "underwater Pangaea" vibe, and the latter with more mermen. I'm not sure how they are really helped by the split, though; while I still find them fun, they still seem a trifle weak to me. That may just be because I'm a bad player, though; I have trouble with the AI on Normal.
It's also worth noting that LA Lemuria is the old Soul Gates Ermor (or rather, Sceleria; you'd think that the thaumaturges would have learned after destroying one empire, but no, they did it twice), the same way that Asphodel is the Carrion Woods Pangaea. What I love most of the new additions are the Rus, myself; the Vanarus were added to MA as a precursor to the LA Bogarus, and there are clear plans for an EA Rus as well that will likely be pre-conquest or shortly after conquest by the Scandinavian-equivalents.
EDIT: Oh, and some other major gameplay changes:
1. Magic has been deemphasized in the early game. Between formations for regular units, a different scaling system for magical research costs, and slow-recruitment options for stronger mages, it'll take longer to get to the point where you can drop game-shattering spells for maximum shenanigans.
2. Thrones of Creation. Capture these to win the game, if you have that victory condition enabled. Each one gives you particular bonuses or sometimes penalties, which means you have to decide if you want to take it.
3. The power graphs can be limited in the game settings, emphasizing effective scouting and spying even more.