the WISE sub-system, or how to combine the four core basic classes into pseudo-multiclasses

Willpower for Magic-Use
 Instinct for Thievery
Strength for Warrior
Empathy for Clerical

you decide which core class will be Major, minor, knowledge and bonus

a Major class climbs +10 in 10 levels
a minor class climbs +6 in 10 levels
a knowledge class +3 in 10 levels
a bonus class climbs +1 in 10 levels 

to build a Paladin, you would say
Strength is Major
Empathy is minor 
(Instinct is knowledge
Willpower is bonus)

 while
to build a Cleric would say
Empathy is Major 
Strength is minor 
 etc