Here's a rough idea/draft. This doesn't force an entire rewrite of the spell system, keeps traditional spells superior, and gives casters a broad range of possible abilities.
Gird Spell
A girded spell lasts for 24 hours or until expended. Only spells with a target of You, or damage-inflicting spells with a duration of instantaneous (this needs adjustment) can be girded. Spells with a natural duration of 10 min./level or longer cannot be girded (check this vs mage armor etc). A girded spell uses up a spell slot of its normal level, and the caster cannot cast a normal version of the spell while it is girded.
A girded spell is greatly weakened from its normal effects. All ranges are halved. Radiuses, width of lines wider than 5 ft., and lengths of lines are halved. If a girded spell has multiple effects (ie detect magic), only the first is active. If a girded spell grants a bonus (ie mage armor or shield), this bonus is halved and rounded down. If a girded spell deals a fixed amount of damage (ie chill touch), that damage is halved and rounded down. If the spell deals damage according to caster level, the caster level increment is doubled (ie, fireball 1d6/2 levels). A non-continious (ie fireball, magic missile) girded spell can only be activated every X rounds, where X = spell level (so magic missile can be used every round, fireball every 3 rounds).
A girded spell may be expended to gain the full effects of the spell. An expended spell may not be prepared or cast for 12 hours or until the 24-hour duration of the girding would have expired, whichever is longer.
Possible restrictions - only one girded spell per spell level; max number of girded spells equal to spellcasting ability modifier; max girded spell level equal to spellcasting ability modifier.