Roach Coach only got one major role in an episode, his debut in Insect Inside. Yet he somehow earned a mention in the full version of The Powerpuff Girls ending theme. Combined with him being such an early villain in the series, it's really likely he was meant to have more roles as it went on. I guess the writers had trouble figuring out what else they could do with him, kind of like what happened to Sedusa.
Going further, the reboot's villain The Gnat is very similar to the Roach Coach, being an insect that controls all sorts of robotic vehicles. Although, that leans more toward the reboot's redundancy with its new villains. The Gnat definitely could have just been the Roach Coach instead of a new character.