If you want your primary schtick to be dealing damage in a min-maxed game at level 20, ask yourself this: can I consistently deal over 200 damage per round? If no, your build probably isn't going to be that strong, and you should either look for a different primary schtick or change your build (btw, 20d6 damage averages out to be 70 damage). For spellcasters specifically, direct damage spells are usually pretty bad and take a back seat to spells that severely hamper or kill your opponent (save-or-die/suck spells) or battlefield control spells like Walls and Fogs. Of course, if you want to deal direct damage with spells, there are builds that do that.
Vow of Poverty is really bad at level 20. Magic Items are just that good. Plus, your spellbook costs money
.
Perhaps the best way for you to abuse Shapechange is to shapechange into a Choker or Chronotyryn (Fiend Folio), giving you extra actions to cast spells. When not actually fighting, take a form with a high dexterity bonus to maximize your initiative.
At level 20, whoever goes first tends to win. Ways you can go first: have a high Initiative mod (see Shapechange, also stuff like Improved Initiative and the Nerveskitter spell help), Contingency->Celerity, Foresight (so you're not flat-footed)->Celerity, Shapechange into a Dire Tortoise (Sandstorm), etc.
Don't worry too much about defense. Usually, the best defense involves killing/disabling your opponent first. For the build below, you have veils for defense, as well as any Persistent defensive spells. You may want to grab a Ring of Spell Battle (I think that's what it's called) for Disjunction defense, or you could just respond to it by disrupting the enemy Wizard (best use of direct damage spells is to interrupt an enemy spellcaster) or eliminating the Line of Effect via a Wall effect (oh look, Wall of Stone isn't affected by Disjunction, and by the rules if a burst effect like Disjunction can't go through obstacles that block Line of Effect).
The most broken gestalt build I can think of that involves a sneaky wizard is
Beguiler 6/Incantrix 10/Shadowcraft Mage 4 // Wizard 3/Master Abjurer 9/Initiate of the Sevenfold Veil 7/Shadowcraft Mage +1 (probably not in this exact order, but hey you're starting at level 20 so I'm not sure it matters). Ban Necromancy on the Beguiler for the Incantrix (lol), and I think you could probably get away with banning Evocation and Enchantment on the Wizard side (if your DM rules that the Incantrix banned school must apply to the Wizard side as well, Necromancy is still a decent choice for that). IIRC
Whisper Gnome (Races of Stone) doesn't lose Int, so it's the best race choice if you have access to it. If you can use LA Buyoff and have the spare experience, the
Draconic creature template (Races of the Dragon p.74) is a fantastic choice for -3000 XP.
If your DM allows you to use both sides of your progression to qualify for PrCs, go with
Beguiler 5/Incantrix 10/Shadowcraft Mage 5 // Specialist Wizard (Abjuration) 3/Master Abjurer 9/Mindbender 1/Initiate of the Sevenfold Veil 7 to grab telepathy and what might be the strongest feat printed,
Mindsight. You can't be Good anymore, but you can still be True Neutral!
.
EDIT TO ADD: You can swap some Master Abjurer levels for Archmage levels without needing any more prerequisites (you should meet the 5 schools prerequisite easily). The Moderate School Esoterica you get at Master Abjurer 7 is pretty useful and the Major School Esoterica is party-friendly, but not great. So you can give up 2 or 3 levels of Master Abjurer for Archmage and snag some stuff like Mastery of Shaping and Spell-Like Ability on your favorite level 9 spell (Maw of Chaos!). Spell Power is also good, and Mastery Of Elements, while not great, gives you some added flexibility (sonic is great) with using direct damage to disrupt spellcasting.
Note: If you're playing with the variant classes from Unearthed Arcana,
use this Wizard variant and drop Scribe Scroll for Improved Initiative.
This build is crazy powerful. You have full progressions of two full casters (the Beguiler's spells aren't so great, but that's OK because you're a Shadowcraft Mage),
full abilities from the three most broken powerful arcane prestige classes, single-attribute dependence (max that Int), and you've had Hide and Move Silently along with other stereotypical rogue skills as class skills for the Beguiler (Able Learner might be a good pick if you want to be rogue-ish, but you will be strained for feats even if you're allowed to use flaws).
Highly suggested reading:
How to Play Batman
How to Play God
Why the Shadowcraft Mage is a Campaign Smasher
Incantrix's Metamagic Effect
The Defenses of the Initiate of the Sevenfold Veil
Feats:
First off, if you can take 1 flaw, take 1 flaw. If you can take 5 flaws, you should probably take 5 flaws. Feats are so good, and you're going to need as many as you can get.
You need to have Spell Focus (Abjuration) by level 3, Iron Will (Beguiler 5 gets you Silent Spell) by the time you enter into Incantrix, Spell Focus (Illusion) by the time you enter Shadowcraft Mage, and 3 levels in Master Abjurer by the time you enter IotSV.
You'll get 4 bonus metamagic feats from Incantrix 10. You need Heighten Spell to make Shadowcraft Mage tick. Extend Spell and Persistent Spell will allow you to make full (ab)use of the Incantrix's Metamagic Effect. Just get max ranks in Concentration (+23), an item of +10 Concentration, decent Con and perhaps some other minor buffs like a Luckstone and you can easily hit the DC42 needed to make a level 9 spell Persistent. Quicken Spell is always a good choice.
You're down 3 feats for prerequisites so you won't have too many left, but here's how you should spend your remaining ones.
If you took that level of Mindbender, grab Mindsight. Perhaps one of the most powerful feats ever.
If you didn't use the variant Wizard option, you can consider taking Improved Initiative or just casting Persistent Heroics (learn with Advanced Learning on Beguiler side).
Rapid Metamagic should be very useful since you'll be casting a lot of Minor Image spells with Heighten Spell.
Versatile Spellcaster from Races of the Dragon is pretty nice. Trade up your low level spells to cast higher level spells.
Earth Sense and Earth Spell aren't required because I think the Incantrix's Improved Metamagic ability applies to Heighten Spell, so you can still get 9th level spells out of your shadow spells. However, these boost your caster level which helps with SR, AND they further reduce your cost of Heighten by 1 so you can cast 9th level spells out of 8th level spell slots. Too bad it only works when you're standing on the ground, which may not be too often at high levels. This is still a solid choice.
Speaking of SR, the Arcane Mastery feat from Complete Arcane combined with any good Caster Level booster that works for SR checks (like the Spell Penetration feat) will allow you to automatically bypass the SR on most creatures. You may just want to use Assay Spell Resistance and save on feats.
If you want to boost your all your spells' DCs, the most efficient way is Snowcasting (Frostburn) + Eschew Materials + Draconic Aura (Energy, Cold) (Dragon Magic) + Dragontouched (Dragon Magic), which gives you +4 DC to all your spells for 4 feats. If you got the Draconic creature template, it only requires 3 feats. If you want more, Cold Focus and Greater Cold Focus gives you +1 or 2 DC on all your spells at the cost of 1 or 2 additional feats.
Able Learner is useful if you want to be a skillmonkey and take advantage of your prodigious Intelligence modifier.
Practiced Metamagic (Quicken Spell) from Races of the Dragon means Quicken Spell will only cost you 2 spell levels. Could be worth it. *shrug*
Extraordinary Spell Aim is pretty funny (read: broken) when used with Anti-Magic Field.
EDIT TO ADD: Even if you're not planning on playing a horribly overpowered character, I hope this has given you some ideas on how to play a powerful Wizard
, and some possible PrCs to take.
