If you are looking at the Deluxe content specifically, the highlight is the .
It introduces a massive, narrative-driven map that showcases the game's late-tier mechanics: high-level spells that can terraform the environment and "Tier 4" units (like Dragons and Giants) that can single-handedly turn the tide of a war. 5. Moral Alignment and Empire Happiness Age-of-Wonders-III--Deluxe-Edition-Repack-Games-Mechanics
Unlike many 4X games where leaders are just stat-boosters, your Leader and Heroes in Age of Wonders III are active units on the battlefield. If you are looking at the Deluxe content