While racial level limits is something of an icky subject -- I mean, what a fun way to talk about eugenics -- I advocate putting caps on the classes across the board. Most iterations of D&D feature an upper-limit on experience, be it 20 levels or whatever. I just want to mess around with 'em a bit.
Not all classes should share the same, generic level cap. Certain professions are more involved than others. The amount of wizardry a Wizard can learn, for example, is far beyond the upper limits of crass thievery a Thief can master. This has always been true, if you really think about it. A twelfth level Thief is just no match for his Wizard counterpart, and that's across all editions of D&D (except maybe 4th). There's a built-in inequality to the classes. While some may view this as a detriment to older versions of D&D, I find this variety in scope and abilities to be part of their appeal. Like real life, it doesn't need to be fair to be fun.
But I digress. The fundamental criterion I'm suggesting for the experience-span of a class, summed up, would be "In terms of accumulated knowledge, how far does it go?"
Here's what I've come up with for the classes of the Hidden Planet setting:
Fighter ~ 12 levels
Thief ~ 12 levels
Wizard ~ No limit*
Mountebank ~ 6 levels
Beggar ~ 6 levels
Fungalist ~ 12 levels
Players have the option to switch tracks and multi-class every time they gain a new experience level. Say, for example, when a Beggar character maxes out at 6 he decides to go Thief. Then after two levels of Thief, he picks up some levels as a Fighter. His current saving throws and to-hit table would be based on the most advantageous values among his three classes.
Another advantage of setting the sights of certain classes lower is that the players of these classes see rewards -- in the form of experience-contingent abilities and features -- faster.
What do you guys think?
*I like the idea that there could be 59th level Wizards doing random/bizarre experiments involving the fabric of time and space.