I finally finished my fanart for this amazing challenge! I was aware that I will have not enough time to make both shikai and bankai separately, so I added them both on one drawing. On the right hand, it is Kenpachi's shikai and on the right one it is his bankai. I couldn't scan the whole image, so only of the bankai's halberd is shown, but I will go and explain what it does.^^
This is my first time using some random background, so don't be too harsh. xD I tried something different, but at the end I managed to do this. Ah well better luck next time.
Special thanks to Hulaberry32 who has explained me on time how to extract the image and place in on a different background. Thank you very much Brooke! :D
And I also want to dedicate this image to my friend granraecero69, who constantly kept telling me to draw something and for this challenge and I finally did. Thank you for that.^^

~Shikai
His shikai takes a form of an unusual barbaric advance sword. The abilities are higher rate in strength and destructive power. He is also able to slash and break opponent zanpaktous thanks to the shape of his own sword. We just a slash he can send a huge wave that can crush all of the hollows! Release name, Hakai (Destruction).

~Bankai
His bankai takes shape of a halberd, with a great gold axe at the top and with the point spear end. When he releases bankai his reiatsu increases, giving him option to attack and harm opponents with just waves of his hands. Everything around goes in destruction, by the yellow dark cloud (from the background). His true form is this halberd, with which he can send a huge and wide attacks from miles and miles away, making it one of the highest reaches zanapktous. His speed also increases as well as his duration, because he gain a special yellow aura around him. Release name, Eikō no hakai (Glorious Destruction).

~Thanks everyone!