I would really like to see the role/specialization of illusionists well developed in this game. A set of well rounded illusionist spells could really expand the gameplay stratigies. Effecting things like-
Diplomancy- increase or decrease how nations feel about each other by clouding the eyes of leaders from other nations.
Defections- cause a hero to defect to your cause-even if you allignments do not match.
protection-even if a hole is knocked into your walls-if no one can detect it (illusionary walls) you are still safe.
Cause panic in other unenlighted cities.
hide outposts or even whole cities with a dome of concealment.
Phantom army-create an army that looks real on the screen but vanishes when attached -thereby diverting enemies resources.
create fake shards- again , they look real on the map until you try to tap into their energy.
-of course each spell should have an appropreate counter.
Any other suggestions or ideas?