Gestalt Prestige Classes
Normally, You may only take one prestige class per gestalt level. However, I'm getting rid of that rule and adding something else.
A creature with a spellcaster class can only gain one effective level in that class per level. This means a creature's spellcasting abilities would never rise above it's number of hit die. A creature with multiple spellcaster classes can gain one effective level in each class per level.
However, classes that let you gain an effective spellcasting level in two or more classes, example: Mystic Theurge in DMG, are still banned from being taken.