For conjuration: I think it will be cool if I can summon deamon. Just use some deamon model and set the stats to be very high. (20 hell knights are very nice, but seeing one deamon slaughtering your enemies will be more fun)
BTW, wouldn't you think 20 hell knights * 5 per battle is too... powerful magic? I mean they are hell knights not peasants, 100 hell knights will surely crush any enemy and it will be no fun fighting a battle that you will surely win
For support magic: 1, how about mirror images? make about 3-5 player mirror images with the same stats but will perish if be hit. very useful if the player is using bow
2, a magic that will boost your strength or agility 50 points, only three times per battle (warrior with >150 strength or >150 agility is a nightmare)
3, invisible magic and higher invisible magic, as the name says, the latter one allows you to attack enemies without losing the invisible status
4, charm humans, can turn your enemies to be your fighters
5, curse, any one shot by this magic lose 10 strength
6, haste, all warriors around you gain 50 agility when the magic is cast
7, bull's strength, all warriors around you gain 20 strength when the magic is cast
8, divine protection, you gain 300 armor points for head and body for 5 minutes
9, horror, you run to your enemies and they are frightened away
For destructive magic: 1, POWERFUL METEOR STORM! kill every one in range, both friendly units and enemies.
2, fire wall, make a fire wall in front of you, damaging people passing it
3, poison, enemies lose 10% of his life point every second for 5 seconds
4, tornado, randomly throw your enegies to somewhere in the map
5, arrow rain, 100 arrows come from nowhere and strike your enemies!
I know most of the magics may not work. Just provide some of my ideas
P.S. the screenshots are awesome, maybe it is not impossible to make an official M&B II: magic world ;P