Here's a boss concept for Giygas of the Mother series.
Well, I've thought of Giygas serving as a sort of power source for the operations of the villains in the game. He was stuck in the Devil's Machine for most of Earthbound to contain his tremendous power. Perhaps the villains take advantage of this and hook up his machine as a power source.
At some point in the story - preferably one featuring Ness -, Giygas breaks out of his machine somehow and appears in his normal "form".
As I've said before, this battle should be anything but normal. You can't directly attack Giygas - his form morphs and distorts in the background. But Giygas can attack you. He can take the form of your allies - albeit with a unique effect where anyone he mimics consists only of a dark red outline and a sort of mist forming within where there is normally solid body. He also attacks you with his trademark "You cannot grasp the true form of Giygas's attack!", complete with text box and a flash of light that inflicts heavy damage and knockback on the opponent.
You can't even try to attack him. If you attack just once, a countdown starts and your characters begins glowing a dark red hue. The more you attack, the quicker this countdown passes. At the end of the countdown, the characters transforms into his/her trophy form and subsequently shatters, pieces falling every which way.
And the only way to defeat him is to refuse to fight. No matter how many times Giygas pummels you, you must remain still. If you do so, your character begins praying to whomever they befriended or love. For example, if Mario is praying, the cutscene shows all of the Toads in the Mushroom Kingdom suddenly start praying. Each individual prayer deals heavy damage onto Giygas.
Eventually, Giygas dies in a very horrific way,
as seen here at 7:16.
This boss fight completely goes against any lightheartedness I want in SSB4's story. But it's just
so awesome.
Plus, he would have
the most disturbing boss music to ever touch Smash Bros. Seriously, don't listen to this if you want to sleep tonight.