Want a grittier campaign with less power creep and so called class disparity? I have had some good results with the following simple system.
1.) Use a stat build system, keep points within a sane range. This keeps players on an even playing field.
2.) Use a static number for hit die results (high or low average) This rewards those players who invest in con, and avoids dice cheating.
3.) Let players play whatever pc class or classes they want until 4th level. Even encourage power gaming up to this point, because things are about to get rough.
4.) After 4th, all players can only advance as Adepts or experts. Thats it.The game plays out very differently from this point on.PCs tend to cling together and work together better because their world just became a very scary place! And the good part is they still have the skills and basic spells to survive.
5.) Be sure to be aware of your pcs adjusted CR, and it's also a good idea to make disposable magic items like potion/sscrolls readily available for purchase to fill in the gaps of useful spells/abilities.
6.) Avoid the temptation of adding the npc warrior class to the above options, unless you want a campaign dominated by a martial type