One of the great voids in Dungeons and Dragons is the absence of a spell-countering or wizard-dueling mechanic.
The current magical combat system is rolled in with the martial combat system, which is derived from the wargames antecedent to D&D.
Using that mechanic, spells are cast during that player's combat turn, and the effects are applied shortly thereafter (unless a saving throw negates the spell effect).
Saving throws, as a way of countering spells, is a free (to the defender) and adequate mechanic, for those spells that affect a character, but what of those spells for which no saving throw is permitted, or for spells that do not affect a member directly? Currently, there is no way to counter those spells within moments of their casting. One must wait until the next round to attempt some kind of response to the previously-cast spell. Unfortunately, at that point, it is often too late to actually counter the effects of the first spell.
Here's an alternative. Any time a spell-caster invokes a spell, any other spell-caster can use their turn (assuming they have not already taken their action) to counter the invoked spell.
In order to do so, the second spell-caster must either have memorized the same spell, or have a memorized spell sufficiently similar (or opposite) in effect that it can be used to counter the effect of the first spell cast by the attacker.
Since the second caster is reacting, in real time, to the initial spell, there is a chance that her spell-countering will fail. Rolling 2d6, the second caster must roll an 8+ to successfully counter the first spell, but that is modified (to the defender's benefit) by the number of levels the defender is above the attacker's level, and by the level of the spell being expended in that defence. I say to her benefit, because if the defender is a lower level, or uses an inferior spell, she still gets the minimum 8+ chance to counter. Call it the Harry Potter effect.
For example, the attacker, a 3rd level MU, casts darkness, a 1st level spell. The defender, a 6th level MU, uses her turn to cast a 2nd level spell, continual light, to counter. Since the defender is 3 levels higher than the attacker, and is casting a spell 1 level higher, the defender gets to add 4 (3+1) to her roll to successfully counter.
Whether or not the defender is successful, that memorized spell used for countering is gone from her memory, and she has used her turn for that round.
But what of situations where the defending MU have already taken her turn or wishes to counter multiple spells? In those cases, some alternative spell-casting cost could be imposed (for example, the MU takes d6 of fatigue damage for each additional spell she wishes to counter, should she have already taken her turn).