The Default Terrain Types for factions would be nice if they weren't all the same (All grassland for Kingdoms, all deadland for Empires, Boring) but here's an idea, why not have spells that morph the terrain instead? Age of Wonders 2 had this, and it would allow players to choose the aesthetic direction of their Kingdom or Empire to better reflect their Sovereign and preferences.

This could also lead to tactical advantages further down the line, different terrain types could be updated to confer different bonuses and penalties to spells, movement, and unit types. Accessory items such as a "Desert Survival Kit" could be added, as well as spells to enchant units to negate negative effects or imbue positive effects on terrain types.