So, I got around to thinking about M20's Magic Attack Bonus. You have Melee, Ranged, and Magic bonuses - Melee for hitting folk, Ranged for shooting folk, and Magic for... what? There's already a way to work out the DCs for your spells, and any Touch spells use Melee Bonus, and ranged ones Ranged Bonus... so what does it do? I know Clerics use it for Turning Undead, but Wizards seem to get the short end of that stick.
Greywulf, M20's creator, suggests a whole bushel of things you could do with it, and briefly mentions the concept of "duelling".
Well, I wanted to expand that a bit. It's something that older D&D made kinda weird (with Counterspelling and Reversible Spells), despite it being a core element of the genre it so slavishly emulates. So, a quick and easy way to run a magical duel!
The two sorcerers faced off, while the battle raged around them. Kutz first encased himself in a shield of ice, a foot thick - but a deep blue flame from Anakaris' hand left it a puddle on the floor, scorching Kutz in the process. Next, Anakaris performed a weird, twisting gesture, and even those fighting were distracted when three live snakes, each two-headed and several feet in length, flew from his hands, seemingly from nowhere, across the battlefield towards Kutz. Focusing his mind and steeling his soul, he threw a ball of telekinetic force their way - scattering them into the already busy melee around about them. He felt that he might have overexerted himself, but steeled his resolve and prepared his next abjuration.
Kutz realised this was going to be a lot harder than he thought.
By forgoing their action on their turn, a Magic User can engage another Magic User in a Magical Duel.
On each of their turns, both combatants roll a d20 and add their Magic Attack bonus - whoever scores higher wins that round. Each round costs the combatants 1HP, and each round they succeed, the do damage equal to their Magic Attack bonus to their opponent. A draw roll means both opponents take 2HP damage. An opponent can leave a duel at any time, but takes the opponent's Magic Bonus as damage (as though they had scored a successful hit).
Both players and GMs should make their descriptions vivid and entertaining - but appropriate to the level of the combatants. A 1st-level mage and a Kobold Shaman will be throwing sparks and stings at each other, while 20th-level mages and demigod wizards will have battles that look more like Rush album covers.
Example Above: Kutz (5th-level Mage) decides to enter into a Magical Duel with Anakaris (7HD Serpent-God Wizard). He rolls 1d20+Magic Attack Bonus, and rolls a total of 15. He declares his action as a shield of ice, which he will expand to engulf Anakaris. Anakaris rolls a total of 17 - he blasts away the ice with a ball of flame, and Kutz takes 8HP damage.. On Anakaris's turn, he rolls a total of 21, and the GM declares he launches snakes from some otherworldly place at Kutz - but Kutz also manages a roll of 21, and negates it by throwing the snakes away from him with the power of his mind. Both Kutz and Anakaris take 2HP damage, from trying to exert more effort than the other.
I figured this would give a few advantages - spellcasters can tie up enemies by duelling them, leaving their allies free to focus on other threats or gang up on the enemy spellcaster. However, it also takes you out of your role as a support character in fights - spending your hitpoints keeping others busy, rather than helping your allies. Sometimes, though, it's worth the risk.