I believe this should be done.
Suggestion:
1 - Character Level and Class Level become separated.
2 - Character Level determines the universal base Stat growth (the highest amount of any Stat that is common to all Classes/Jobs) and aggro radius.
Currently, Class represents 100% of your naked Stats.
With my suggestion, Character represents the portion of your naked Stats that is compatible with all Classes.
The Class adds the "specialized" Stats (because each class has a different Stat growth besides the Traits).
Example:
At level 50, if the lowest base Vitality among all classes is 191, that is what Character Level provides you, while any amount above that is provided by the respective Class Level.
3 - Class Level determines Abilities, Traits, bonus Attributes, Equippable Items and any Stat growth above the universal (provided by Character Level).
4 - Some quests would require Class Level, others might only require Character Level.
5 - You only gain Character Experience when gaining Experience with your highest Class.
Reasons:
Immersion - A high level Hero that wants to train a new class shouldn't suddenly feel like a weakling. All the physical and mental conditioning as well as battle experience you already accumulated should mean something - more than just Cross Class skills.
Watching your HP go down by 95% when you swap classes is just sad.
Harmless - It does not affect any of the synched content as both Character and Class Level are synched.
It merely affects leveling experience, allowing you to keep a feeling of progression throughout your whole playtime.
Example:
Synched Content
No noticeable difference, because Character level is also synched.
In Sastasha where requirement is Lv15 and synch 18:
Scenario 1: A Level 18 character with level 18 class (18/18) is as strong as a 60/60, because both character and class cannot exceed 18.
Scenario 2: A 60/15 is as strong as a 18/15, because both character and class cannot exceed 18.
Scenario 3: A 18/15 is stronger than 15/15, but weaker than a level 18/18, and maybe also weaker than 17/17 and 16/16 in some situations depending on differences in traits, skills and gear.
Non-synched Content
The only content where it makes a difference.
1 - Outdoor enemies will "recognize" your higher character level by allowing you to come closer without attacking you.
2 - Base stats provided by a high level Character make you stronger than a lower level character with the same Class level.
This doesn't mean you will have the stats of a level 60 character as a level 1 Class, since
a) the gear you equip depends on your Class Level
b) part of the base Stats come from the Class.
c) Traits depend on Class level
d) Bonus Attributes depend on Class level
You will feel more powerful than a newbie, but it makes sense, since at that point you're not exactly a new guy anymore.
In pure gameplay terms, it will merely make it easier and faster to level up, but that "difference" starts fading as your class level gets closer to your character level.
It doesn't make or break anything that really matters.