For the Judge Dredd one, you could just refine it:
Title: "Judge Fear - Motorbike Design"
Description: "In a brutal future world, law enforcement need the latest equipment to dispense justice and survive. An armed and armoured police motorbike that is the perfect tool for a future cop."
Brief: "Design the motorbike. It must have clear police markings as well as armour and weapons."
You might want to specify no background or character, if you want to stop things getting too complicated for a weekly, and keep it to the bike.
Following on from that, you could do a character based one.
Title: "Judge Fear - Character Design"
Description: "In a brutal future world, law enforcement need the latest equipment to dispense justice and survive. From body armour to hand weapons."
Brief: "Design a future cop, male or female. They must have clear police markings as well as armour and weapons."
(You could make it more like game/film design and say draw one pose of the character and paint three different variations of their outfit?)
Gallery:
http://community.imaginefx.com/fxpose/thulsadooms_portfolio/default.aspxWebsite:
http://www.wdlee.co.ukBlog:
http://duncansguide.blogspot.com