I think the final season of SU could have benefitted from having a villian that Steven would have had to fight. No redemption, no talking them out of it, no friendship, he would need to fight. It would be a final lesson within the series: When all else fails, fight. Not everyone can be reasoned with.

"I don't think you understand, Steven. The power of Talk no Jutsu doesn't work on a being of pure evil."

"But Cartman can't be pure evil! Surely he has enough good in him for him to change his ways!"

"Steven, did you know that Cartman arranged for a teenage boy's parents to be murdered, ground them up into chili meat, and fed them to said teenager, causing him to understandably have an emotional breakdown and be humiliated in front of his favorite band?"

"!!!"

"Didn't think so."