We didn't always have critical and armor peircing, and for a while I've been chewing on an idea of a similar vein. At some level, (it could be an existing level, or one that the cap has yet to be raised to) players gain a stat called "Combo rating" Every time two or more wizards on the same team cast the same spell on the same round at the same target, there will be a chance that they will cast the spell together, determined by their combined Combo rating. That is, they will all do the casting animation simaltaniously, and a single spell will be cast that combines all the damage/heal modifiers from all participating parties, with the total damage/healing output being multiplied by the number of casters, plus a 33% boost for every caster minus one. All additional effects that are applied to the caster are applied to all casters, but without any special bonus. (drain spells will give the same amount of health to all casters barring wards that affect incoming heals.) The way the probability of a combo is calculated is that the first wizard to cast a particular spell uses his combo rating as a percentage chance that the combo will start, and everyone else casting that spell at the same target (and hasn't cast it yet) uses their combo rating as the chance to join in. If no one joins, the combo is ignored and there will indeed be no way to even know if it happened. If any of the casters fizzle, they will start to cast the spell with the rest, but will fizzle and be excluded from the combo.
