This is an attempt to make a D&D wizard class that works like a GURPS wizard. I don't claim that it's perfectly balanced. The salient points are:
Learns spells from a book, like a wizard and not a sorceror
Doesn't need to prepare spells
Can cast unlimited spells per day, subject to Fatigue
Spellcasting may fail
As an arcane spellcaster, is subject to arcane spell failure
All rules are as for a normal wizard (hp, saving throws, BAB, weapon proficiencies, familiar, skills, feats, etc) unless noted below.
Spell casting
Ignore the Wizard spellcasting table.
A GWiz can cast spells of spell level up to half his level, rounded up (eg a 5th level GWiz can cast 3rd level spells).
A GWiz has a Spellcasting modifier of GWiz Level + Int Mod (eg 8 for a 6th GWiz with 15 Int). To cast a spell, he rolls against the DC of the spell. A 1 is always a failure, and possibly a fumble. He is NOT allowed to Take 10.
The DC to cast a spell is 11 + (3 x spell level). So a GWiz needs to beat DC 11 to cast a Light or 17 to cast Web.
While the GWiz doesn't need to prepare spells like a normal wizard, he needs to revise regularly. Each full day between revising the spell from his spellbook and casting it increases the DC by 1. The Spell Mastery feat (PHB) avoids this for the selected spells.
Any attempt to cast a spell costs fatigue equal to (spell level x 2) - (caster level/2) + 2, rounded up, minimum = spell level. So the 6GW above loses 4-3+2 = 3 fatigue when casting a 2nd level spell. A critical check (20, roll again) on the casting roll means half fatigue (round down, min zero); a fumble check (1, failed roll again) means double.
Metamagic feats can be used as required, but they increase the level of spell for both cast chance and fatigue.
Armour affects casting chance: each 5% increases the DC by 1. This is instead of the usual Arcane Spell Check.
Specialist G-Wizards of a given school get a +4 specialist bonus to spellcasting and use 2 less Fatigue (minimum = Spell Level/2, down) when casting spells from that school only. They are barred from other schools as per a normal specialist.
New metamagic feat: Spell Expert: pick a school (inc Universal) to get a +2 expertise bonus to spellcasting and use 1 less Fatigue (minimum = Spell Level) when casting spells from that school only. Can be taken multiple times, for differing schools. Stacks with Specialist.
Fatigue is expressed as a loss of STR points. If casting a spell would take the GWiz to 0 or less STR, the GWiz cannot cast the spell. If this happens by a fumble, STR stays at 0 and the excess comes off his CON.
STR is recovered at 1 point per 10 minutes, CON at 1 point per day of rest. [Lesser] Restoration can be used to aid recovery of both. The Endurance feat doubles recovery rates for both.
Examples
A starting 1st level human GW with 16 Int, with the feats Scribe Scroll, Endurance and Spell Expert-Enchantment, has a modifier of +4 or +6. He needs to beat 11 to cast Light (taking 2 fatigue), 14 to cast Shield (4 fat) and 14 to cast Sleep (3 fat). He wears leather armour, so these DCs are increased by 2.
A 7th level human GW with 17 Int has 6 feats, including Expert-Evocation, Expert-Enchanter, Expert-Transmuter. He has a modifier of +10 or +12. He needs to beat DC 11 to cast Read Magic (no fat), 17 or less for Flaming Sphere (2 fat), and 23 or less for Charm Monster (6 fat). Unhappily, his spellbook has been stolen. As he hasn't revised for a week, all these DCs are increased by 7.
An 18th level Illusionist GW with 24 Int has many feats, including Expert-Illusion. He has a spellcasting modifier +25 or +31. He needs to beat DC 38 to cast Gate (11 fat), Weird (9 fat) or Quickened Silent Phantasmal Killer (9 fat).
Magic Items
Magic items to go with the G-Wiz include:
Powerstones:
supply mana to power the spells. 1 mana substitutes for 1 fatigue at the owner's option. A powerstone recharges naturally at the rate of about 1 point per day to its maximum capacity.
A powerstone can theoretically have any capacity, but more than about 15 is extremely rare and powerful. Cost is about 200 x capacity^2 gp. Some stones can be used only for certain spells or schools of spell; these are somewhat cheaper (say, 30 x cap^2 for a single spell, or 100 x cap^2 for a single school).
A G-wizard can use only 1 powerstone at a time. If two stones are within 6' of each other, only the more powerful will recharge.
Foci:
A focus (typically an amulet, tattoo, talisman or wand) aids the G-wizard's memory and gives a Focus Bonus to spellcasting. The nature of the bonus may vary by spell (eg, Magic Missile), spell theme (eg, any Cold spell), school (eg Abjuration) or all magic.
The focus might give a bonus of +1 to +6. A G-wizard can possess may foci but use only one at a time.
Cost is typically 2000 x bonus^2 gp for a general focus, 1000 x b^2 for a school or 300 b^2 for a spell.
