I think I've figured out power sources...

[erf_beto]

there's already a variant in UA that does something similar: Magic Rating http://www.d20srd.org/srd/variant/magic/magicRating.htm

Essentially, every class has a Caster Level progression - even fighters and rogues! - and they stack when multiclassing. Of course, a fighter can't use his caster level unless he multiclass. You could go further to say wich caster levels could stack: Arcane, Divine, Nature, etc, so a Sor10/Wiz10 has an arcane caster level of 20, but a Sor10/Drd10 would have an arcane caster level of 10 and a Nature caster level of 10 as well.

When it comes to 4e, I guess every Power Source will have its "caster level" equivalent - wich I think is the OP's intention. But I dont think Base Attack Bonus is actually power source dependent. Martial sources will probably give Initiator-like levels, not BAB, so fighters can make special combat manouvers that will outshine wizards with the same BAB.

Maybe each class will grant bonuses through progressions like:

Excellent 1:1 / Good 2:3 / Bad 1:2 / Poor 1:4

(Or after seeing how Saga has done the Half-Level thingie, maybe every Power Source Caster Level will be at half-level, and maybe Excellent means 'half class level', so for a Wiz10, 'half character level' + 'half class level' = full level 1:1 progression...)

That way, we could describe classes as:
Wizard: Arcane PS (Excellent), Every other PS (Poor)
Cleric: Divine PS (Excellent), Every other PS (Poor)
Fighter: Martial PS (Excellent), Every other PS (Poor)
Rogue: Martial PS (Good), Every other PS (Bad)
Paladin: Martial PS (Good), Divine (Bad), Every other PS (Poor)

And later on:
Druid: Nature PS (Excellent), Divine PS (Bad), Every other PS (Poor)
Psion: Psionic PS (Excelent), Every other PS (Poor)
PsyWarrior: Psionic PS (Good), Martial PS (Good), Every other PS (Poor)

This way, Paladin can multiclass into Cleric or Fighter and better customize his power sources (increasing divine spells or martial manouvers), without gaining useless abilities at higher levels, and still remain a paladin at heart. The same will not happen if he gets into wizardry - he's got to start from the beggining then!

These are wild guesses, but with some polishing (and better writing)... who knows?

 

[Thornir Alekeg]

Another streamlined possibility is that all classes get the same BAB progression, which is applied to all attack forms. The difference could show up in the damage.

A wizard could pick up a sword and hit the goblin with it, but do very little damage compared with his arcane attack. A fighter of the same level can hit just as well, but pummels the goblin with his sword for a lot more damage than the wizard can do with it.

As levels increase, BAB progresses, but the melee damage bonus of a Fighter progresses faster than a Wizards, but on pace with the damage from the Wizard's arcane attack.

 

[jmucchiello]

No, the disparites would be far too great for attack bonuses.

Unless you change the way AC works.

 

[Simplicity]

Actually, the more I think about it, the more interesting this gets.
Now, instead of worrying about "caster level", you worry about Base Arcane or
Divine Bonus.

Paladins have a slower progression of their base Divine Bonus.
Spellswords (or whatever their fighter/mage class is) get slower marital and arcane bonuses.
Spell effects are based entirely on the modified bonuses (base + wizardly implements).

Spell level could pretty much go away. You simply need a certain bonus to cast a spell
(aka "Wizards can cast 25th level spells").

Example, a 5th level wizard might cast fireball (a 5th(!)-level spell) and get 5d6 damage.
A 10th-level spellsword can also cast fireball and get 5d6 damage. (assuming 1/2 progession, which I'm not sure is a good idea).

You no longer need to break up the spell lists, you just have requirements in the spells (this much base Arcane bonus and/or this much base Divine bonus).

 

[skraig]

I suspect there'll just be two different advancement progressions (like Saga, though with 30 levels to cover they may juggle the numbers a bit) - let's call them Good & Poor.

Fighter has Good (Weapon) Attack Bonus, Poor everything else (for the purposes of multiclassing). The Wizard, Good (Spell) Attack Bonus, Poor (Weapon) Attack Bonus. The Fighter/Wizard simply adds up the attack bonuses as per usual, with the Fighter levels providing a (albeit "Poor" progression) bonus to the Wizard's Good (Spell) Attack bonus.

Further, I see the Wizard's, Warlock's and Cleric's "Good" Spell Attack bonuses all stacking if they multiclass.

I don't think that is how it will work. One of the goals of 4th ed has been stated as having "multi-classing work with every combination every level"
This sort of setup would make a fighter/mage pretty weak. In fact it would have most of the same problems fighter/mages have now.