A few adjustments that would make Multiclassing more balanced and usefull;
Favored Class: XP penalties no longer exist. If a race has a favored class, it now has the ability to take the level in its favored class when it reaches 90% of the XP needed for next level; his XP total "Jumps" artificially to the next requirement if he chooses his favored class. This means that humans will automatically level slightly faster under ordinary circumstances, as will Half Elves, compared to members of other races who don't have that favored class. Optionally, unusually weak races(goblins?) could have a lower "Jump %" and a reduction in CR as they gain levels.
Fractional Bonuses: If a bonus provides a modifier per set number of levels, then it now provides a fraction bonus. For example, a Wizard gets half a BAB point every level, and a third of a Reflex save point every level. When multiclassing, the fractions add together, allowing a character to keep pace if his classes all have the same modifiers.
First-Level Bonuses: Each class gets certain bonuses they only receive if they enter the class at first level; normally this is only starting gold and skill points, but with this change, it includes the following: +2 to good saves, Fighter bonus feat, Call Familiar(Becomes feat), Scribe Scroll. Any class with "Frontloaded" abilities will have only -some- of its abilities at 1st level, and others will require feats to access for someone who multiclasses. For some classes, the PC might get a choice of which to take; a Barbarian speed or Rage ability, for example; and then have to get a feat to get the other, or have to wait for 2nd/3rd level.
Spellcasters: Any time a Spellcaster takes levels in a non-spellcasting class, with every 2 non-spellcaster levels he gains a +1 bonus to caster level in the spellcasting class. If he takes levels in two different spellcasting classes, then only the second of the two he takes will be able to add bonuses to the other. A spellcaster's caster level can never excede his class level for spells in general. If the chosen class is Bard, then each 2 caster levels added add on a third, and if the chosen class is Paladin or Ranger then he can continue his progression normally at caster level = class level. (I.E: A Fighter 10/Sorcerer 10 would effectively cast spells as a level 15 Sorcerer, and a Fighter 10/Bard 10 would effectively cast spells as a level 17 Bard. A Wizard 10/Cleric 10 would either be 15 wizard caster level or 15 cleric caster level.)
Result:
Multiclassing won't grant you any unusual power compared to your allies, but it also won't cripple you if you make a "Poor" multiclassing choice for in-character purposes. Your Druid/Ranger won't shoot himself in the foot by losing all those spellcaster levels, nor will he have unusually good saves.
Favored Class: XP penalties no longer exist. If a race has a favored class, it now has the ability to take the level in its favored class when it reaches 90% of the XP needed for next level; his XP total "Jumps" artificially to the next requirement if he chooses his favored class. This means that humans will automatically level slightly faster under ordinary circumstances, as will Half Elves, compared to members of other races who don't have that favored class. Optionally, unusually weak races(goblins?) could have a lower "Jump %" and a reduction in CR as they gain levels.
Fractional Bonuses: If a bonus provides a modifier per set number of levels, then it now provides a fraction bonus. For example, a Wizard gets half a BAB point every level, and a third of a Reflex save point every level. When multiclassing, the fractions add together, allowing a character to keep pace if his classes all have the same modifiers.
First-Level Bonuses: Each class gets certain bonuses they only receive if they enter the class at first level; normally this is only starting gold and skill points, but with this change, it includes the following: +2 to good saves, Fighter bonus feat, Call Familiar(Becomes feat), Scribe Scroll. Any class with "Frontloaded" abilities will have only -some- of its abilities at 1st level, and others will require feats to access for someone who multiclasses. For some classes, the PC might get a choice of which to take; a Barbarian speed or Rage ability, for example; and then have to get a feat to get the other, or have to wait for 2nd/3rd level.
Spellcasters: Any time a Spellcaster takes levels in a non-spellcasting class, with every 2 non-spellcaster levels he gains a +1 bonus to caster level in the spellcasting class. If he takes levels in two different spellcasting classes, then only the second of the two he takes will be able to add bonuses to the other. A spellcaster's caster level can never excede his class level for spells in general. If the chosen class is Bard, then each 2 caster levels added add on a third, and if the chosen class is Paladin or Ranger then he can continue his progression normally at caster level = class level. (I.E: A Fighter 10/Sorcerer 10 would effectively cast spells as a level 15 Sorcerer, and a Fighter 10/Bard 10 would effectively cast spells as a level 17 Bard. A Wizard 10/Cleric 10 would either be 15 wizard caster level or 15 cleric caster level.)
Result:
Multiclassing won't grant you any unusual power compared to your allies, but it also won't cripple you if you make a "Poor" multiclassing choice for in-character purposes. Your Druid/Ranger won't shoot himself in the foot by losing all those spellcaster levels, nor will he have unusually good saves.