Honestly, I don't think a single caster class is the way to go. Just as Fighter, Paladin, Ranger, and Barbarian all have a place as a warrior class, so do Wizard, Sorcerer, Cleric, and Druid as casters.
The reason the warrior aspect of multiclassing works well is because BAB isn't on a separate table for each class -- it stacks. Spellcasting is more like saying that a Monk/Fighter multiclass can only use the BAB he earned as a Monk for unarmed attacks and can only use BAB earned as a Fighter for swordplay. If things worked that way, there would be (appropriate) cries of "broken".
So, to fix the spell progression, we need a mechanic for caster level. I have a couple of ideas, although they are from the top of my head and I make no claim to them being a suitable final solution, they should make my point. I'll give my best idea, half-baked though it may be.
Each character literally has a "Caster Level" (CL) value similar to the BAB. Wizards, Clerics, Druids, and Sorcerers all get a +1/1 progression (like a Fighter's BAB); Bards get a +3/4 progression; and Paladins and Rangers get a +1/2 progression. The CL from these classes stacks, just like BAB does.
This CL is referrenced against a chart that looks suspiciously like the Wizard's "Spells/day". This gives the _total_ spells slots the character has available to them.
The hard part is how to populate the slots -- and this is where the half-baked part comes in. Can a Wiz1/Clr19 add Wish to his spell book? Can a Clr1/Wiz19 spontaneously swap Magic Missle for a Cure Light? -- What about Dispel Magic for a Cure Serious? How does the Sorcerer's artillery barage work?
I think the Sorcerer is the easiest. For every Sorcerer level taken, the character gets a few "spell-like abilities" of her choosing (with some level restrictions). These really are spell-like abilities except that there are still spell levels attached to the spells and they have an odd means of determining "x/day". Every day, when the Sorcerer prepares her spells (for all classes), she sets aside a number of slots into a common pool for her abilities. The return on investment for these slots is 2:1, thus the increased castings the sorcerer enjoys.
I see four possibilities for determining Wizards.
-1) Base it on just Wizard level: every two class levels gives access to another spell level. The Wiz1/Clr19 can only cast lvl 1 Wizard Spells and a Wiz5/Clr15 can cast 3rd level Wizard spells.
-2) Base it on just Wizard level: every class level grants access to another spell level. A Wiz1/Clr19 only has 1st level Arcane Spells, but a Wiz5/Clr15 has 5th level spells. This provides a bit more flexibility for casting but, since the slots aren't immediately available to cast the higher level spells doesn't effect a single-classed character at all.
-3) Add an exclusive skill for arcane spells, or for each school of spell. Base max level of spell available on the Total of the skill. 2nd level spells require a 7 (6 ranks +1 from an Intelligence 12). 5th level requires a 15 (12 ranks +2 from intelligence +1 from an assumed magic item).
-4) Similar to 3), add "points" in the class that can be applied to knowledge of the schools. This should be balanced to provide balanced access to a pure wizard, and work out well for customizing specialists, too. It would also allow a Cleric to only dip into Illusion spells and do it well. For balance, the different schools would probably have varying costs, much like they do for specialization. This one probably has the most promise for balanced play, but could prove troublesome.
Clerics.... Options 1) and 2) from Wizard would work fairly well. Another route to go would be to limit breadth rather than depth. Make the most use of the Domains mechanic. Throw every Cleric spell into a domain and give Deities favored and standard domains. The Favored Domains would work like current domains, while the Standard Domains just allowed spell selection. Give the Cleric a handful of domains (Favored and Standard) at 1st level. Award more as they progress in Cleric.
Of course, you could just turn Clerics into divine Sorcerers, which actually makes more sense to be, anyway.
I'm rather stumped on Druids. My instinct is to use the Sorcerer mechanic rather than preparation.
Paladins and Rangers would reflect Clerics and Druids, respectively. Bards would reflect Sorcerers.