I would like to see many of the existing spells tweaked to be more useful. I think this would make dueling much, much more interesting and less predictable. Also, my suggestions are to tweak current spells, not to add new ones, so should take a relatively small amount of work on the dev's part. Here are some ideas:
All spells: Have spell level make a bigger difference in the damage they inflict. Right now, being 20, 30 or 40 in Red, Crystal or Dark Ways have only a small increase in effect on spells like Missile, Arrow, Flame Burst and Weakness. I think a weak spell should be somewhat strong when cast by a high level mage.
Freeze: Increase the effects of this spell. Cause the victim to be slowed to half speed. If cast on them again, they should be frozen in place for a short time.
Weakness: Allow it to be cast from a longer range than just touch. As it is, you have to stand face to face with the victim for a very long time to complete the spell, and it's impossible to survive that unless your opponent is a very weak noobie. Have the effect be to significantly lower the damage your opponent does (this might already be in effect, but the "touch range" makes it useless so nobody uses it).
Darkness: Have the black cloud be locked on the target instead of the caster, causing them to have hampered vision. Set the spell to last 20 or 30 seconds. Could be very interesting and fun in RPs.
Meteor: This spell currently requires 40 Red Way to cast... yet the max you can train to is 20. Potions must be used to raise Red Way so you can cast it. Even though it is a level 40 spell, it does the same damage as an Energy Arrow (level 20), and takes about 5 times longer to cast. I suggest keeping the slow spell time, but make it do a crap-load of damage, maybe even killing in one hit, since it takes so long to finish casting.
Rock Armor: Right now, it seems quite useless. Have it give a defensive boost, maybe raise armor level temporarily by +10 or +15 or so. Also possibly subtract slightly from magical skill of ways other than Brown to make things more interesting.
Defensive Wind: Possibly have the defender take half damage or even no damage on a successful "save" from magical attacks.
Electrotouch: Cause a significant amount of damage. Considering that it doesn't last long, and only works with melee attacks, it should cause a lot of damage, of course dependent on how trained the character is in the appropriate Way.
Flame Burst: This is a nice spell, but I think it should do more damage from casters with high Red Way.
Flame Spire: Cause more damage.
Stone Fist: Allow us to train high enough to cast this spell. Since this is a very high level spell, it should be powerful. I recommend causing a huge damage bonus from melee attacks (making it useful for someone who wants to play a "monk-type" character).
Flying Stones: This is a higher level spell. It should to a significant amount of damage. I think it does about the same damage as an Energy Arrow, but is much slower to cast. I think that slower-casting spells should do higher damage to make them useful.
I think tweaking these spells, making them more useful, will have a dramatic improvement on dueling, and make duels far more interesting.
-Roahn