OK, I thought about it some more and I still fail to see the bad imbalance here. Note that "effective
class level" doesn't mean "effective
character level"! I think there still are plenty of reasons to stay single-classed in most cases. Basically, this system would allow you to trade 1, 2, 3 or 5 levels of class features in your highest class for 4, 8, 12, 15 levels of class features in another class at 20th level. By now, I do think that for most spellcasters
and martial adepts, 20th level class features in a single class
are comparable to 19th and 4th or 15th and 15th level class features in two classes, with neither one being the obvious contender.
I don't think it completely falls apart at lower levels either: The sweet spot where multiclass characters are closest to overpower single-classed ones is probably at 4th level, where a character can have 3rd level class features (which often are just as good as 4th level ones, i.e. 2nd level spells/powers) from two classes. The single-classed character's fewer class features will be slightly more powerful though, and the single-classed character will always get access to the next level of spells/powers sooner, i.e. a foo 2/bar 2 is probably slightly stronger than either a foo 4 or a bar 4, but on the other hand, a foo 3 or bar 5 is probably slightly stronger than a foo 2/bar 1 or foo 2/bar 3, too.
For the system to work out, power probably
needs to increase disproportionally as class level increases, which should be the case for most casters and martial adepts most of the time, so I'm not too worried about any of them anymore, even bards, clerics and druids, especially below level 17. It's still worth keeping this in mind when redesigning the magic system, though.
I am a bit worried about barbarians, beast master rangers, shapeshifter rangers and rogues. For barbarians, the additional abilities while raging might just be enough, but I'm not sure, it depends on how fast they accrue/grow. For the rangers, I'd consider not stopping their spell progression prematurely and maybe giving them a fifth level worth of spells, which could include something along the lines of Cure Critical Wounds, Find the Path, Mass Cat's Grace, Mass Owl's Wisdom, Stoneskin, Summon Nature's Ally V, Transport via Plants. For rogues, I don't think bonus skill tricks are enough. Maybe sneak attack should grow faster then linearly or something. Capstones can help with this at level 20, but there probably should be something fast increasing all the way.
Otherwise, I'm afraid that for these classes there would indeed be little reason to stay single classed: A rogue 18/wizard 2 or rogue 16/barbarian 4 would probably be almost striclty better than a rogue 20 and a rogue 10/ranger 10 would probably be better than either rogue 20 or ranger 20, as the loss of high levels in those classes doesn't offset gaining a disproportionate number of lower levels in another.
On a sidenote, the system should probably include a provision that features granted by multiple classes such as an animal companion increase at a rate of beast master druid class level plus beastmaster ranger class level plus half the sum of all other class levels, rather than something silly like effective beastmaster druid class level plus effective beastmaster ranger class level or having two animal companions at the effective class level of each respective class.
Alternatively, if the slight increase in the overall power level and balance shifts are unacceptable in return for the added flexibility that the system gives you, I'd suggest scrapping most of the effective class levels idea and using a simpler system that just addresses the core problem of spellcaster multiclassing:
- Non-spellcaster-nor-martial-adept classes multiclass as by PHB, which works reasonably well as all classes improve your BAB, HP, saves and skills.
- Martial adepts multiclass by ToB, which also works reasonably well as their initiator level is the sum of all levels in their martial adept class plus half the sum of their levels in all other classes.
- Spellcasters determine their caster level just like martial adepts determine their initiator level, by adding together the level in their spellcaster class plus 1/2 their levels in all other classes, which should make them work reasonably well.
- A class' initiator/caster level is capped at double your actual class level in that class. (That's a change from ToB.)
- Initiator/caster level determines the highest level of maneuvers/spells available and the number of maneuvers/spells known. (That's also change from ToB.)
- Everything else is based on actual class levels, including, but not limited to, the number of maneuvers/spells readied, the number of stances known, the opportunities at which you can swap an old maneuver/spell for a new one and the number of (base and bonus) spell points and any other class features.
Compared to the "full" effective class levels system, this would have less power creep and would make multiclassing in general a bit less attractive, both of which should help balance and single-classed characters, if either of them was considered to be a problem otherwise.
Compared to your "new version" of the effective class levels system, this would take the importance of multiclassing in a specific order (a wizard 16/rogue 4 wouldn't be weaker than a rogue 4/wizard 16) and would help actually getting rid of dual-class advancing PrCs (a wizard 10/wizard 10 wouldn't be weaker than a wizard 5/cleric 5/mystic theurge 10), both of which seem like flaws in that proposal to me. Also, as the special multiclassing rules only apply to martial adepts
Or do use the "full" effective class levels system and have some things (such as spell points) be determined by actual class levels, just other ones than in your "new version". I strongly think that at least initiator/caster level, the highest level of maneuvers/spells available and the number of maneuvers/spells known
need to depend on effective class level for the system to work as intended.
Or massage either system a bit more by limiting your maximum effective class level to 3/2 times your actual class level. Or by making off-levels in different classes worth different amounts. For example, levels in other martial adept/spellcaster classes would increase initiator/caster level by 1/2 each, but levels in other classes would only increase it by 1/4.