Anything that add new mechanics - I'd bet the first one we will have will be about transport ships and trading routes/pirates/naval combat. Other ideas:
-Religion and its structures - adding temples, shrines,monasteries,conversion and a piety system (I know, very CK, but it would be awesome)
-Banditry - Ambush - roguery quests
-New mounts/trained animal troops/pet system
-Economy overhaul - more trading enterprises, new taxes/laws, guilds
-New weapons and new cultures(eg Japanese/Chinese factions with flails,katanas,kusari gama,etc)