You need to check every "solution" against these three cases, at minimum:
Wiz1/Clr19 (Or Clr1/Wiz19)
What should this character's total spell package look like?
Bbn10/Clr10
Two classes with no bonus feats, one non-caster, one full caster, in an even split.
Wiz10/Clr10
I think most folks agree that this caster should have the functional power of at least Wiz15/Clr15 (Mystic Theurge power level).
I like to gut systems and rework them from the ground up, as I did with
Elements of Magic (Revised, and Mythic Earth).
My ideal solution would be for Pathfinder to be a class-based system, but with the core 12-ish classes covering nigh all the powers you'd need in a game. You build those classes from the ground up to provide 3.5-esque gameplay, and to use the same core mechanics, but you can fix all the problems. As with many Pathfinder fixes I would like, though, the bulk of the gaming public says they don't want them because they don't want the crunch in their dozens of books to become obsolete.
Now, we can either keep all the existing classes in place and try to make a fix that works using those classes, or we can be a little outside the box.
My whacky proposal
Make the core classes good. Real good. Good enough that you really don't want to multiclass out of them. Indeed, the core classes, and other 20-level classes like them from various other sources, are intended to be primarily used as your character's whole career arc.
The trick is that for those who want to come up with their own concept, we have a "build yer own" class. More on that in a moment.
Now, some people want to mix concepts. You can multiclass freely, and we'll use the Base Caster Level idea that's been bandied about for those wanting to be cleric/wizards and such.
For refresher, for each spellcasting class, you take that class level, and add 1/2 of all your other character levels to determine your spells per day and spells known (but that cannot exceed twice your class level). Your actual caster level (for determining variable effects) is always equal to your character level.
A Cleric 10/Wizard 10 has the spell access of a 15th level cleric and 15th level wizard, but with 'caster level' of 20 for both.
A Cleric 2/Wizard 18 has the spell access of a 3rd level Cleric and 19th level wizard, with 'caster level' 20 for both.
That's the stop-gap solution. It's workable, but it's not great for all classes, and it really doesn't, y'know,
feel like you ever learn to combine your two power sources. You're just a little of column A, and a little of column B, with no interesting synthesis.
Where we get interesting, now, is by adding a new class. The Pathfinder class.
Pathfinder
Where other heroes pursue established roads to power, you find your own path.
Philosophically, in 3.5 there are three styles of power characters pursue: skill, battle, and sorcery.
- Skill involves defeating challenges through clever exploitation of your environment, your allies' strengths, and your enemies' weaknesses.
- Battle involves harming your foes with weapons and magic, and protecting against the same.
- Sorcery involves influencing the world and breaking the normal rules of reality.
Mechanically, there are three things that characters develop as they grow in level: power, finesse, and resistance.
- Power is how good you are at affecting people. 'Affecting' means hurting, or forcing them to do something, or successfully reshaping reality. (Represented by Str and Cha)
- Finesse is how good you are at avoiding being affected. (Represented by Dex and Int)
- Resistance is how good you are surviving when you're affected. (Represented by Con and Wis)
When you take the Pathfinder class, you decide your two arrays. For both style and mechanics, you can either pick:
1. Good at one, average at another, poor at the third; or
2. Good at two, untrained in the third; or
3. Great at one, poor at another, untrained in the third.
So you can make, say, a swordmage who is good at battle and sorcery, and poor at skills; and who is good at power, average at resistance, and poor at finesse.
Or you could make a meditative telepath who is great at sorcery, poor at skills, and untrained in battle; and who is great with finesse, poor at resistance, and untrained in power.
A fighter is basically great battle, poor skills, no sorcery, with good power, good resistance, and no finesse. A blaster-based mage is great battle, no skills, poor sorcery (in the sense that while he uses magic, he mostly just uses it to hurt stuff), with great power, poor resistance, and no finesse. A 'warlord' would be good skills, good battle, no sorcery, with good finesse, average power, and poor resistance.
Now just what do these ratings mean?
Well, you'd divide character abilities into whether they are skills, battle, or sorcery, and just have a big ol' chart of what's what. And each ability would have an associated character level requirement. You can take things at that level if you're good at that style, or at that level +x if you're average, or that level +y if you're poor, and never if you're untrained.
At 1st level you pick a couple abilities, and then another at every X levels thereafter, building up a unique suite of powers. Stuff like "fighter focus" would emulate the bonus fighters get, and "sneak attack" would give you sneak attack. "Fire magic" would give you access to spells with the fire descriptor. Having more than one "magic" ability doesn't give you more spells per day, but it gives you more versatility.
How highly you rate Power determines your base attack bonus and what caster level you use for damaging attacks. Finesse determines what sorts of defensive abilities you can take (evasion, monk's AC, etc.), the caster level for your manipulative magic, and the extent of your skill tricks. Resistance determines how many good saves you can have, your hit die, and your caster level with defensive magic.
I just came up with this idea off the top of my head, so it's still a little loosey-goosey, but I'm sure I'll come up with a way to fit it all together.