A class is just a collection of themed moves that work together to create a certain set of abilities and qualities that give the class their unique feel. If you’re up for it, creating a new class is the next natural step along the way. Your first consideration should be how the class relates to the existing classes. No character exists in isolation, so you should think carefully about why this class is different.
An excellent first step to creating a new class is to think about what fictional characters you’d like to take inspiration from. Don’t slavishly follow what that fictional character can do (after all, they weren’t in Dungeon World) but use them as a guide for what’s so cool about that character.
The inspirations for the classes in this book are fairly clear, and made clearer by the notes in the margins. Note that not every inspiration is taken entirely: the wizards of Discworld inspired the slightly pompous style of the wizard, but the wizard is far more competent and casts spells more like a wizard from Vance’s Dying Earth. The inspiration is one of style, not an attempt to recreate what a certain character could do in a certain book.
With a clear idea in mind you have a few basic steps that aren’t a concern when writing single moves: HP, Bonds, Look, equipment, alignment, races.
A class’s HP is some base+Constitution. Base HP is almost always 4, 6, 8, or 10. Having more HP than the fighter and paladin will take the spotlight away from those characters unless you’re careful. Having less HP than the wizard is probably character suicide. 4 base HP makes for a class that is deliberately fragile, they’ll need help from others when the swords come out. 6 base HP is for classes that aren’t ready to fight, but can at least take a hit. 8 base HP is enough to take some hits and get into combat a little, while 10 base HP is for skilled warriors and those who have no fear of battle.
Damage is chosen from the dice available: d4, d6, d8, d10. The classes presented here all use a single die with no static bonus, but there’s no reason not to experiment with other options: 2d4 or 1d6+2, for example. High HP and damage tend to go together, but your new class could be a pacifistic brick wall or a glass cannon—fragile but dangerous.
Alignments show the starting outlook of the class. Most classes will have Neutral as an option, since only the most dedicated classes are so tied up to an ideal that the self can’t come first. A good alignment move is something that happens with some regularity and guides the player to a particular type of action they might not otherwise consider. An alignment that happens as part of the normal course of play, like “When you gain treasure…,” doesn’t really show the character’s ideals. Adding some requirements, maybe “When you gain treasure through lies and deceit…,” adds an element of ideals. Now the alignment says something about the character (they prize pulling a con on the unsuspecting) and requires the player to think about how they play. Alignment is a telling fact about the class in the world, too. Everyone knows that paladins are supposed to be paragons of Good and Law, right?
Bonds are where the class’ outlook shines through. It’s the place where you, the designer, will most clearly interact with the player at character creation. Unless the class is particularly social or antisocial, write four bonds. If the class is very connected to others, add a bond; if they’re cloistered, remove one. Avoid bonds that dictate a moral or ethical stance but do think about how your class interacts with their allies—the thief steals things but helps protect the party from traps, the fighter defends his allies and kills monsters that might harm them, the wizard knows secret knowledge and shares or hoards it. You can use the rules for writing new bonds as a starting point, but avoid including proper names in starting bonds.
Look is largely left to your imagination. This is an excellent spot to think about your fictional inspiration. What did they look like? How could they look different? Including at least one choice about clothes helps establish style without making the player think about buying clothes.
The equipment choices should always include at least one weapon option and one armor option unless the class is clearly lacking in fighting skill. Dungeon rations are also pretty much required; a starting character without food going into a dangerous area borders on stupid.