Warrior - Martial Combat.
Thief - Thief Skills, and Backstabbing.
Wizard - Spellcasting, and Rituals.
Cleric - Healing, Rituals, and Turn / Rebuke Undead Creatures.
Whatever things each class is best at: Their primary abilities. Instead of using a flat BAB per level, as in D & D 3E forward; allow the Bonus Die Roll, which scales slowly with class level. Let the bonus be variable like the d20 roll itself is.
I know that DCC purists will say you shouldn't do such a thing; but every edition and variant of D & D sees experimentation, or has mechanics borrowed and then applied to other games. I'm speaking more to the latter.
In other words, an excuse to use some of those funky dice outside of DCC.
The d3, d4, d5, d6, d7, d8, d10+ progression would quickly get more people into the funky dice world.
I shouldn't be left to rot in this sickness all alone....