Everyone likes a dual nines build. Also, the Bard/Druid archetype is really popular from what I hear. Understandable, really, it does date back at least to the Greeks, what with music that soothes wild beast and whatnot. If only we didn't have to take that pesky Bard level. Of course, getting Inspire Courage would still be really neat, but we can't have everything, can we? Actually, we can.
Assumptions:
*Flaws are not allowed.
*All other, non-variant 3.5 (including Dragon mags) is allowed.
Without further ado, the build!
The Nice Guy Next Door With the Really Cool Dog
Human Druid 7/Heartfire Fanner 3/Sublime Chord 2/Arcane Hierophant 8
Druid 1 - Apprentice (Entertainer - grab Perform as a class skill), Knowledge Devotion
Druid 2
Druid 3 - Southern Magician
Druid 4
Druid 5
Druid 6 - Natural Spell
Druid 7 - Skill Focus (Perform) from Heward's Hall
HFF 1
HFF 2 - Melodic Casting
HFF 3
SC 1
SC 2 - Song of the Heart
AH 1
AH 2
AH 3 - Words of Creation
AH 4
AH 5
AH 6 - Dragonfire Inspiration
AH 7
AH 8
Knowledge Devotion and Apprentice give you all skills you need as class skills. You need Diplomacy, Listen, Spellcraft, Perform (any), Knowledge (Arcana), and Profession (Astrologer), but not all require full ranks, so you should be fine as a Human with 10 Int. Note that HFF gives 6+Int skills, so you can use those levels to catch up if you've been slacking on any (HFF doesn't get many skills it ought to get as a casting PrC, though - talk to your DM). We need to set two feats on fire to get class skills for requirements, but we have enough slots anyway. Anything after level seven isn't mandatory and can be customized at will. Since it gets Bardic Music, I just thought I might as well add the old IC Optimization retinue.
Okay, but how are we qualifying for Sublime Chord and [dual-progression PrC]?
With that handy little feat named Souther Magician. Alternate Source Spell also works, but it, too, requires the ability to cast arcane spells.
Note: If your DM allows you to read "Able to cast Xth-level arcane spells" as "Able to cast Xth-level or higher arcane spells", then our job got much easier (and a feat slot was saved), as you can then drink from a Dragonsblood Pool once per year, get an arcane 3rd-level spell slot, qualify for everything in one fell swoop, and live happily ever after. However, for the remainder of this thread, I will assume this is not the case.
This build gets dual nines from good lists, Wild Shape as a Druid 15 (meaning up to Huge size), a Companion Familiar as a Druid 15/Wizard 10 (If I'm reading the AH's ability right, you don't seem to actually need to have a familiar for it to gain the familiar bonuses. If I'm wrong, just take Obtain Familiar in there somewhere, if you want it.), Bardic Music (including all abilities thereof) as a Bard 10, +14 BAB (+15 with partial BAB), and +9/+5/+17 base saves (+9/+9/+12 with partial saves), and other goodies from diverse class features. You can also cast Glibness on your familiar companion and have a better Wild Empathy right out of the box, if you have it tell the right lies.
Furthermore, this is playable from level one, starting with a strong Druid chassis, progressing into HFF (which costs you class features, but no casting and what you get makes up for it), and only losing two caster levels to Sublime Chord, but we get high-level, fast-progression arcane spells out of the deal, so that lessens the sting. Finally, we get to doubly progress our double awesomeness from ECL 13 through 20.
I'd like to think it's also fairly DM-friendly, using no house rules, variants, or alternate systems. If Dragon mags are not allowed, you can use Virtuoso instead of Heartfire Fanner, but you'll then lose out on Inspire Courage, Heartfire awesomeness, and another caster level.
Finally, it's easily customizable, as all you need are three feats (four, really, but Skill Focus is gained without spending a slot). If your DM allows you to customize class skills, you can eliminate the need for two of those, and if he lets you read "Able to cast xth-level arcane spells" as "xth-level or higher arcane spells", you can ditch SM, just visit a Dragonsblood Pool once per year, get a 3rd-level arcane spell slot, and meet all your prereqs in one fell swoop.
The build suffers from Wis/Cha MAD, but... eh, whatever. Since you're already Cha-based, you can opt to dump one AH level and the music feats for a Sacred Exorcist dip to get Divine Metamagic.
If you're really into Bardic Music (like, you value it over Wild Shape and animal Companion), you could take five more skills and go Fochlucan Lyrist instead of Arcane Hierophant. You'd end up with BM as a Bard 18 and +16 BAB. (and Bardic Knowledge as a Bard 10, for what it's worth...)
All in all, this is my take on the Druid/Bard archetype, without trying for any more brokenness than what spellcasters already come with.
Non-core Sources
[spoiler]Heartfire Fanner: Dragon 314
Sublime Chord: Complete Arcane
Arcane Hierophant: Races of the Wild
Apprentice: DMG II
Knowledge Devotion: Complete Champion
Southern Magician: Races of Faerun
Heward's Hall: Complete Scoundrel
Optional:
Dragonsblood Pool: Complete Mage
Virtuoso: Complete Adventurer
Fochlucan Lyrist: Complete Adventurer
Sacred Exorcist: Complete Divine
Melodic Casting: Complete Mage
Song of the Heart: Eberron Campaign Setting (might cause cross-setting problems if your DM's anal about it)
Words of Creation: Book of Exalted Deeds
Dragonfire Inspiration: Dragon Magic[/spoiler]