Dragon Magazine 200 had an article on changing the description of a spell, making customized wizards much easier to design. Most of the time the mechanics of the spell remain the same though a couple of the example spells show some changes. I have an idea that goes farther- change the school of a spell. Obviously this doesn't work with all spells and Illusion and Necromancy are more or less excluded (Illusion really is hard to justify). Beyond that, the sky is the limit. And this isn't really a new idea. Consider the changes that were made when the 3rd edition was released. In the 1st and 2nd editions wall spells were Evocations, not Conjurations. Necromancy included healing and other positive spells that affected the soul and body. And so on. Now consider how the idea might be expanded.

Wall of stone could be created by Abjuration: a anti-mineral shell effect, Alteration: turning air to stone, Conjuration: summoning the stone from a quarry or other plane, Divination: knowledge on how to manipulate the ground and force the stone to emerge, Enchantment: charm tiny earth spirits to do the work, Evocation: melt soil and shape it into a wall, Necromancy: animate dead animals and plants and have them create the wall. (Since it was part of two schools, I listed both for the appropriate edition.) Some castings and effects will be faster than others and there should be some differences in the walls created.

Animate dead is harder to justify using all the schools, but there are a few- Conjuration and Evocation can summon the energies needed to power the dead and Alteration can make the bodies partially constructs or animate them in a similar manner to animated objects. Obviously this can have a major impact on turning attempts.

Fireball is a bit easier than animate dead- Abjuration creates a barrier that allows heat to enter but not escape, Alteration turns a rock or other object into a bomb, Conjuration summons flames from the plane of fire or collects all the fire within a 10 miles radius, Divination shows the caster how to heat the air with minimal effort and Enchantment uses (again) small elemental spirits. Necromancy might cause a spirit to ignite and Illusion can do squat- you can make someone think they are on fire, but it doesn't cause the burns.

The schools could also be used as mutation modifiers for Mutant Future and similar games. Most of the time, the result is just a change in the visual or auditory description, but there can be some rather odd effects:

Energy ray- Abjuration creates a barrier that filters energy and provides immunity to that energy for d4 rounds. Alteration causes the target(s)' bodies to partially turn into that energy. Conjuration draws the energy from somewhere, potentially impacting another place or person/creature. Enchantment is very different- it causes the target to be attracted to the energy. Necromancy replaces the energy with dark matter or anti-matter, avoiding most forms of immunity. Illusion attacks the target's mind. Divination is useless and Evocation is the current effect.

Quick Mind- Abjuration removes distractions from the mutant's mind (and makes it harder to get their attention). Alteration causes minor changes to the artifacts studied or being repaired. Conjuration allows the mutant to find the best parts or non-obvious flaws. Evocation provides a power source for the mutant to safely use while working. Illusion is a 3D map of the artifact and is the result of a mental scan. Enchantment and Necromancy are useless and Divination is the effect.

Even though this idea has so much merit, DMs and GMs should be wary, at least for spell design, as it obviously impacts niche protection for most spells and specialist casters. Something that a setting designer may want to include to provide some balance is the gall. Magical galls are side effects that impact objects and are the result of spell effects outside their standard school. They can be as simple as changes in color or texture to destruction or transmutation. Unless a spell is designed specifically for it, galls are not positive. In other words, they are useless or dangerous unless the spell is a higher level than usual.