Neoclassical Geek Revival has its five classes and the pie piece system to mix and match between them. Life is fine and dandy, however as always remember that I built the system as a toolkit you can plug and play into your game, and that doesn't just mean the content but how the content is structured. Lets take a look at the classes "Warrior, Wizard, Rogue, Bard, Priest" : these lump everything I want in a swashbuckling fantasy game. But I don't use wizard and usually don't use priest when I play the sci-fi game I run with ConstantCon.
Perhaps you want a different type of characters and challenges? It is easy to swap out classes. If you wanted to say add in "the athlete" for an ancient greek setting or "the fool" for a more fairy tale game you could. Each class follows a different framework. The reason I say "swap out" rather than add is that five is kind of a "magic number" where a player cannot (no matter what) start out with one piece in every class, but if that is their ultimate goal they could achieve it and have one pie piece in everything by tenth level. Assume we are removing "Rogue" for these examples.
1.) It has its own modifier:
This modifier is given to people who do not have the class too, that is important. Everyone can do this, people with the class do it better. The modifier is given in the same way as the other class: 1/3rd your level if you not part of the class, 2/3rds of your level if you have one pie piece in the class, your level if you have two pie pieces and your level plus your milestones if you have three pie pieces (technically your level plus two per milestone if someone manages to get four pie pieces at tenth level). This score is also modified by a single attribute. This attribute should not be shared with any other class modifiers your game is using.
Athlete's improve Fitness, modified by Health
Fools improve Chance, modified by Luck
2.) It has five powers, and a sixth special power
These powers should be unique and awesome benefits the class gives, try to avoid one "awesome power" and a bunch of crappy filler. That is really hard. Also remember that the modifier from step one should be useful beyond these powers. Having an "Algebra" modifier for "Mathmagicians" if it is only useful if someone takes the "Abra-Calculus" power.
So Athlete could recycle the Rogues power of "Parkour" without any difficulty. You might also put in a skill like "Endurance" where they take less damage from wilderness travel, or "Sprint" where they move faster. A fool might have skills like "Blissful Ignorance" where they cash in luck at a two for one rate if they aren't aware they are in potential danger, you could recycle the "Jack of all trades" skill and rename it "lucky break" to work the same, perhaps a skill to allow them to add their Chance modifier to saving throws. The Rogue power "Opportunism" would work just as well for A fool with a rename.
3.) It has a personal item:
Every class has a "personal item" that grows in ability when the class completes some act that is the point of the class. Warriors who fight get a trademark item, wizards gain a talisman, priests relics etc. This has a corresponding result on the 2d6 that should have a 1/6 chance of coming true, or 1/3 if it is a very uncommon occurrence (currently wizards finding spells). Each of these die combinations should be unique in your game. If Bards need to roll 10+ for their personal item, nothing else should use exactly the same combination (10+) though it can use something that occurs on 10+. Priests need to roll doubles, meaning a bard/priest who rolls boxcars would improve both items. Since we are swapping out Rogues in this example the cheater method is just to keep "7" as the die combination.
So the Athlete has a "Trophy" that improves whenever he wins events or solves a problem through a particularly epic task of physical fitness (like shoving over a building or swimming across a raging sea). Each benefit level from the trophy could give a +1 presence bonus to appeals or something else, I am just wining it, figure out what you want it to do. The Fool could simply recycle the "Lucky Item" the rogue uses.
So there you go, always remember that Neoclassical Geek Revival was build not just as a completed product, but as something you can rip apart and plug into your game. It is a toolbox above all else.