I'll fix your wagon! I'll mow your lawn! I'll clean your gutters!
Maybe it's just me, but the saying "I'll fix your wagon" doesn't sound like much of a threat. Observe:
"I'll fix your wagon!"
"You know, now that you mention it, one of the rear wheels on Johnny's Radio Flyer is a little loose."
"Uh, well...I..."
"As long as you're here, you can paint the house and clean the bathroom."
"Uh, this isn't exactly going the way I planned."
"You should've thought about that before you offered to fix my wagon."
"Grr."
"Less grumbling, more fixing, please."
"I'll fix your wagon!"
"You know, now that you mention it, one of the rear wheels on Johnny's Radio Flyer is a little loose."
"Uh, well...I..."
"As long as you're here, you can paint the house and clean the bathroom."
"Uh, this isn't exactly going the way I planned."
"You should've thought about that before you offered to fix my wagon."
"Grr."
"Less grumbling, more fixing, please."