In AD&D there were some spells that were part of two schools- Glyph of Warding was abjuration and evocation, Fire Shield is evocation and alteration, Leomund's Secret Chest is alteration and conjuration/summoning, Project Image is illusion/phantasm and alteration, Spiritwrack is evocation and abjuration and Prismatic Wall is abjuration and conjuration/summoning. There is one spell, Guards and Wards that is in three schools- evocation, alteration and enchantment/charm (and I can see adding conjuration/summoning as a fourth school).
Thinking about a new series of defensive spells brought me back to dual school effects. For this series I am taking Wizard Lock and adding something that would be in another school. All of them mention a failing of the Wizard Lock. This means anything other than Dispel Magic, Anti-magic Shell, Wish and the weakness to more powerful magic-users.
Defensive Door- this 5th level spell conjures a 2 ton block of stone if the Wizard Lock fails. The block fills the doorway and is permanent.
Offensive Door- this 4th level spell evokes a web of lightning in the doorway if the Wizard Lock fails. It does 1d4 points of damage per level of the caster and there is no save for those who pass through it. An 8th level version adds that anyone slain by the lightning is animated as a zombie.
Bone Door- this 6th level spell turns the door into a mass of bones. If the Wizard Lock fails, the bones animate as 1 superior skeleton per caster 3 levels. A superior skeleton has 6 hit dice, is immune to Dispel Magic and is turned as a wight but is otherwise the same as a standard skeleton. They are permanent and free willed after killing those who broke the lock.
Door of Horrors- this 6th level illusionist spell causes those who break the Wizard Lock to see a phantasm of their loved ones dying. Those who fail their save versus spells are shaken and suffer -2 to hit and damage for one hour per caster level.
Door of Riches- this 5th level spell reacts to the failing of the Wizard Lock by transforming all the metal in the room beyond it into Fool's Gold (as per that spell).
Door of Slides- this 6th level spell reacts to the failing of the Wizard Lock by creating a 10' X 10' pit in the room beyond that leads to the next level in the dungeon. Some casters put climbing monsters down there as added security.
Another idea that I am considering is adding Detection spells to offensive effects. This becomes a filter that raises the spell level by 2 or 3. Whatever the spell detects is the only alignment, race, substance, whatever that takes damage.
