Rather than a single path, I suggest a system of various penalties and perks for wizards. I feel this more varied approach to it falls in line with the class theme as well (no two wizards are the same). This should be available only to specialist wizards.
The penalty a wizard would take would be having to choose a second forbidden school of magic. Any time he attempts to cast a spell of this school, it simply fails. The wizard would have freedom to choose his own forbidden school, ideally (I really hate how we can't already).
As for the boon he would receive in return, one of the following. (These are just ideas, some may be overpowered, or just ridiculous, or maybe even not strong enough for the loss of a second spell school.)
Improved Specialization: These are only available to specialist wizards, and affect their chosen school.
-Enhanced Evocation: Evocation spells have their caps raised by 5 levels.
-Enhanced Conjuration: Summons are summoned with a few basic buffs on them. Bull's Strength, Endurance, Cat's Grace, Magic Fang... whatever is deemed sufficient to counter out the loss of Transmutation.
-Enhanced Divination: Various divinations provide bonuses (or increased bonuses) to Spot, Listen and Search (enough to bring them on par to characters with these as class skills). Clairaudience provides 5/+1 DR to balance the loss of Ghostly Visage.
-Enhanced Necromancy: Create Undead becomes Create Greater Undead. Better damage on negative energy spells.
-Enhanced Transmutation: The Polymorph spells provide additional buffs to physical stats, allowing wizards to reach the +12 caps in polymorph forms. Eagle's Splendor provides a scaling bonus to Bluff.
-Enhanced Illusion: PhK and Weird apply fear on a failed will save. Illusion spells provide bonuses to Hide/Move Silently (enough to bring them on par to characters with these as class skills, at least). Color Spray deals low levels of random elemental damage (perhaps like burning hands).
-Enhanced Enchantment: Scaling, permanent bonus to Appraise. All enchantments that cause paralysis or similar effects apply a bleeding effect (or perhaps something similar with slightly increased damage). Dominate spells provide the effects of Magic Fang (or its greater counterpart at higher levels) to the affected creature.
Alternative Perks:
-Improved Wand Use: 50% chance of retaining charges on wands. Or, if this is too weak (I know there's infinite use on FL), infinite charges on wands.
-Mystic Theurge: The Wizard gains a Use Magic Device score of 15, representing his familiarity with Divine Ritual. Piety used when spell components run out.
-Battlemage: The Wizard gains Medium or Heavy Armor Proficiency, and reduced arcane spell failure. This should probably scale with level.
-Spellsword: The Wizard gains bonus magic damage on-hit, along with a bonus to attack. Comparable to a Cleric's Divine Favor, perhaps.
-Improved Familiar: The familiar gains significant buffs, allowing it to fight as well as an animal companion. (Greater Magic Fang, Awaken effects on the familiar.) If possible, the familiar is resized to be larger to reflect this.
-Contingency: The Wizard gains a 3-slot Sequencer, untradeable, useable once per day.
-Spell Penetration: The Wizard either gains all three spell penetration feats, or rolls twice if possible against spell resistance, or something else that makes dealing with spell resistance easier. Perhaps spells of his chosen school ignore spell resistance? I don't know.
Wizard Perks
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Wizard Perks
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