In the recent interview on KTAM radio, the subject of ideas for future DLC came up, so I thought Id get the ball rolling:-
Morale effects, (optional).
1 extra unit per race/faction.
New factions + sorcerer lords to complement these factions + heroes/champions.
Terrain effects on tactical map, (optional).
New resources, world features, monsters, and Titans.
New buildings for each faction, including an extra tier to walls/defenses, etc.
New spells.
New planes. (underground)
Dungeon exploring
Extra unit leveling, once the level limit is reached.(Nothing overpowering, just a few extra hp per level, like in battle of wesnoth) This has the side effect, of keeping you really invested, and interested in units, if theres no level cap, or very high level cap.