First thought was Bowser for my eldest side and Bumble Bee (Transformer) for my youngest but I just couldn't sink my teeth into the idea. It hit me yesterday while watching them play with their Master Chief action figures, why not Halo!?!
Paint colour bought today is 'Canada Goose' by General Paint - sorry I can't get an image of the colour but it's basically gray.
I just love this image and can have it customised to life-sized for 19 pounds each as well as many colour choices. Found via Ebay
This is also a contender and I love the bright colours.
Have purchased this green puzzle light already and think it's a perfect choice for having not knowing the theme at the time.
Exciting! I may spend more time in their room than mine!