Personally, I think the original post is a great solution and addresses nearly all of the problems I've seen with multiclassing.
I also agree with the "double level cap" idea as well.
As for Improved Multiclassing feat, I'd suggest maybe 5 levels of the class or some such. Seems reasonable to say you have to be committed a decent ways into a class before you can "improved" in it. Beyond that, I don't really see any other prereqs being necessary. Even ability scores at first glance sound like a nice prereq, but they can prevent some character concepts unnecessarily (as I found when trying to make some test 4e multiclass characters).
Oh, and for the Improved Multiclass feat, what about a way to make favored class meaningful (since like a great many other, I have never used the XP penalty), maybe you get that feat for free for your favored class?
The more I think about this the more troubling high split classes seem to me. A 18/2 ranger/rogue would have effectively 19 levels of ranger getting all the ranger class benefits except for the last 2 levels plus 6d6 Sneak Attack. The loss of 19th and 20th level capstone powers seems trivial compared to the benefits gained.
That's not a problem with the level cap originally mentioned. The ranger 18/rogue 2 would only gain +2d6 sneak attack instead of +1d6. Seems fine to me.
So now I'm thinking that Paizo should come out with a second book (Pathfinder PHB II?) that just contains 55 definitive combiantions, with rules for each one! I think this would be the only way to balance each and every combo, and it would allow players to follow a specific path for their character concept.
We could have Warrior-Wizards, Fighter-Thiefs, Psion-Monks, Bardic Mages, etc, etc...
Each with its own set of synergistic abilites that combine the two classes in unique ways. Something like a Ranger-Druid would become the ultimate wilderness warrior (protector).
I greatly dislike this idea. Given all of the non-Paizo classes out there, I think a general solution that can be easily applied to WotC classes is by far the best way to go. I think your idea would be cool for an additional product of multiclass variant classes, but there are so many different ways to focus a fighter-rogue for example (half and half, mostly fighter with a little rogue, mostly rogue with a little fighter, burly thug who fights dirty, deadly silent warrior, etc.) that I really dislike the idea of having a set definition for all multiclass combinations.
Certainly, you can come up with some really interesting rules and either make them feats or variant abilities that rely on having X levels in 2 classes. Plus it doesn't address the 3-way multiclass at all, which, although rarer, should be addressed.
But if I compare, although quickly, a Wizard 5/Cleric 5, under Kae's system, to a Wizard 3/Cleric 3/Mystic Theurge 4, I see a big difference in terms of spell levels. In the first case, the PC casts 3rd level arcane and divine spells as a 7th level caster, whereas in the other case, the PC casts 4th level spells as a 7th level caster.
Should this be, or should the list of (IM) abilities actually include spells per day?
DW
Ideally, I think it should increase spells per day and spell level you can cast. Book of 9 Swords works that way, but given the class charts I can see it being a bit more confusing. However, Lisa Stevens has stated that they consider Pathfinder RPG in a way a more advanced and complex system people can graduate to from 4e. So I think some complexity isn't terrible.
And (again with the double level cap), I think increasing spell levels and [i]maybe[/] even all class abilities is still balanced in general. Having more caster level is nice, but with many spells having caster level maximums (i.e. "1d6 per caster level up to 10"), it's still possible to be left behind. With the preponderance of mystic theurge style PrCls from WotC, it seems that they agree.
So a Wizard 6/Cleric 6 would be: (going with even levels since odd levels are suboptimal, and we want to see if overpowered)
Kae's Original Post - Wiz 6/Cleric 6
Caster level 9, highest spell level 3
Allow spell level increase as well - Wiz 6/Cleric 6
Caster level 9, highest spell level 5
Wizard 3/Cleric 3/Mystic Theurge 6 instead
Caster level 9, highest spell level 5
Going all the way to 20 with split it would look like:
Kae's Original Post - Wiz 10/Cleric 10
Caster level 15, highest spell level 5
Allow spell level increase as well - Wiz 10/Cleric 10
Caster level 15, highest spell level 8
Wizard 5/Cleric 5/Mystic Theurge 10 instead
Caster level 15, highest spell level 8
So increase spell level as well sure seems balanced. The only tricky thing I can see if deciding what does and does not increase. With mystic theurge, your familiar, wizard bonus feats, and turning undead would still be at level 3. When you look at all of the base classes and try to figure out what would and won't stack (since only a couple classes have spellcasting as their main ability, of course, and some like druid have spellcasting and shapechanging as two main abilities). However, many later mystic theurge style classes did include new class abilities on top of the dual casting.
So it might be ok to just simply increase all class abilities (everything other than HD, saves, BAB, and skills). I'm thinking of trying it in my campaigns to see how well it works.
This occurred to me on the walk home from work (those gas prices are helping my fitness and brainstorming). It needs some rewording to make it clearer, but within the context of this discussion, hopefully it makes sense.
New feat: Improved Multiclassing
Prerequisite: 5 levels in a base class*
Benefit: When you take this feat, it applies to one particular class. One half (rounded down) of your other class levels and racial hit dice count towards your selected class's abilities, up to a maximum of twice the selected class's level. This does not affect base attack bonus, saves, skills, or hit dice, just class abilities including spellcasting and caster level.
For example, a fighter 5/rogue 4 PC with Improved Multiclassing (fighter), would have the class abilities of a 4th level rogue, and a 7th level fighter. As noted above, base attack bonus, saves, skills, and hit dice would be determined by normal multiclassing rules.
Special: This feat can be taken more than once. Each time it applies to a different base class. Characters gain this feat as a bonus feat for their favored class.
Note: This feat should be used in place of, rather than in addition to dual-class ability prestige classes like the mystic theurge.
* Base class = any class with 20 or more levels.
Thoughts? I tried to make it a more simple alternative that can replace mystic theurge style prestige classes win a very generic way.