Its a trend in south park for the most random side characters to get big roles, and this is why ANY 4th grader could suddenly become relevant for at least a episode.

Lets start with the two obvious ones:

• Heidi- Had an entire arc dedicated to her and Cartman.

• Bradley- Was revealed to be Mintberry Crunch, a alien from another planet.

• Karen- Was a prop for Kennys family, but got episodes all about her and Kenny.

Then we can go even further:

• Clyde- Was a minor character in early seasons.

• Token- Was a background character in early seasons.

And the most obvious one:

• Butters- Used to be a background character and now has the same amount of screen time as Kenny.

This means literally ANY minor character can get some big focus, but that's just a theory.