As you can gather from the title, I am not totally satisfied with how multiclassing works in PF (or DnD 3.X in general). Don't get me wrong - I think that the basic idea of mixing and matching classes as you see fit is utterly awesome. However, it seems to me that this option is clearly suboptimal for a specific segment of the classes in PF - namely the spellcasters. I would like to discuss how to tweak the rules so that multiclassing may achieve its full potential, as it were.
First off, my assumptions: I think (and it has been pointed out to me) that multiclassing is basically fine for warrior/non-casting classes. The reason for this is that combat abilities already stack in a way that spellcasting doesn't, namely in the form of BAB. Imagine for a moment that every class had its own "fighting progression" - BAB, iterative attacks, etc. - that didn't stack across classes. Suddenly, multiclassing becomes a much less viable option even for martial characters.
This, I think, is the root of the problem with multiclassed spellcasters. Spell progressions from different classes do not stack and are incompatible with one another. Also, spellcasting classes will advance your combat abilities, however slowly, while martial classes do not advance spellcasting at all. My suggested fix* is to simply make each class contribute half its level to the spellcasting progressions of each of your classes, if any. I realise that this is not worded very well, so let me give an example. A Ftr10/Wiz10 would cast spells just like a Wiz15, due to 10 wizard levels and five spellcasting levels from being a fighter. The wizard would not contribute anything to the fighter beyond what it does anyway, since the fighter does not have a spellcasting progression. Similarly, a Wiz10/Clr10 would cast like both a Clr15 and Wiz15. This obviously makes the Mystic Theurge and similar classes obsolete. In my opinion, this is a good thing, since I don't think that it should be necessary to take a PrC to achieve this sort of effect.
Note that this idea is a drastic downgrade of another suggestion I had to improve multiclassing. I will restate it here for your evaluation:
I also have a sort of idée fixe concerning multiclassing. A multiclassed character would get something I call “bonus class levels” for want of a better name. It works like this: For each class C, you add together the levels in all your other classes and divide by two, yielding a number n. You then count as n levels higher in class C for the purpose of class features, i.e. everything that is listed in the rightmost column of the class's progression table. Good lord, that sounds complicated :)To give an example: Rasha is a Ftr6/Wiz6. He gets half his wizard levels as a bonus to his fighter levels for the purpose of class features, and vice versa for wizard. Thus, he would have the saves, skills, BAB, hp and regular feats of a Ftr6/Wiz6, but the bonus feats, weapon and armour training etc. of a Ftr9 as well as the spellcasting ability of a Wiz9.
I think this would give multiclassing a nice bump (I am a fan of MC), but it might be overpowered. I honestly can't say where this character would stand compared to a straight Ftr12 or Wiz12.
I am actually still rather enamored with that idea, but it has been pointed out to me on another forum that a Brb6/Ftr6/Rgr6 would be ludicrously overpowered compared to a Brb18 or Rgr18 under this system. I am at a loss how to prevent this sort of hyper-stacking of class abilities especially among the martial classes, which is why I developed the toned-down system above. I would very much appreciate any input you have for me.
* I am aware that the word "fix" implies that something is broken and that everyone might not agree that this is the case. This is purely from my perspective.