Seems like there are enough combo PrCs... I'd make it a Feat tree.
Spellthief [Reserve]
Prereqs: Cha 13+, Ability to cast a 1st level arcane spell, 1d6 Sneak Attack or Sudden Strike
Benefit: By sacrificing 1d6 sneak attack or sudden strike damage when making an attack against a spellcaster, you can steal a spell from him, up to the highest spell level you can currently cast (or your Charisma bonus, whichever is lower). You can "store" stolen spells, up to a number of spell-levels equal to your Charisma bonus. You can cast stolen spells or use them to power feats which require the expenditure of a spell.
Energy Spellthief [Reserve]
Prereqs: Spellthief, ability to cast an Evocation spell of at least 2nd level
Benefit: By sacrificing 2d6 sneak attack or sudden strike damage when making an attack against a creature with energy resistance, you can steal 10 points of energy resistance from him. If you have a 4th level or higher Evocation spell available, you can steal 20 points of energy resistance instead. If you have a 6th level or higher Evocation spell available, you can steal 30 points of energy resistance instead.
Penetrating Spellthief [Reserve]
Prereqs: Spellthief, ability to cast a Divination spell of at least 4th level
Benefit: By sacrificing 1d6 sneak attack or sudden strike damage when making an attack against a creature with spell resistance, you can steal one point of SR for every two levels of your highest available Divination spell.
Wardthief [Reserve]
Prereqs: Spellthief, ability to cast an Abjuration spell of at least 2nd level
Benefit: By sacrificing 1d6 sneak attack or sudden strike damage when making an attack against a creature, you can steal an effect currently affecting it. The effect you steal must be dispellable -- it must have come from a spell or spell-like ability, not a supernatural ability. You cannot steal permanent effects.
.... or something like that. The basics would be to promote Sorcerer / Rogue combos (or Bard / Rogue combos) because there's already such insanely great synergy between Wizards and Rogues.
Cheers, -- N