The last major expansion before the 2026 Theater Packs, and it's entirely about the Pacific: it overhauls Japan, splits Nationalist China from Communist China, adds the Philippines, and takes naval warfare up a level with aircraft carriers.
What New Content It Adds
The pack splits its additions between four Pacific nations and the naval mechanics that finally give that theater an identity of its own.
Nations: Japan, the Two Chinas, and the Philippines
Japan gets an overhauled tree with the north-south tension far better developed. 
Nationalist China and 
Communist China get separate, considerably deeper content than before, when they shared most of the tree between them. The Philippines debuts as a playable nation in its own right.
Mechanics: Carriers and New Doctrines
The faction system expands with more internal management options, new military doctrines arrive built for the Pacific theater, and naval mechanics around aircraft carriers and their embarked aviation take a substantial leap forward.
Is It Worth It?
If you're interested in Japan or China, it's a straightforward buy. But even playing in Europe, the carrier and naval aviation improvements show up in any game with a strong naval component — of the major expansions, this is one of the ones that best ties country content to system mechanics.