Mecha/Full Metal Sion aka Durge Skin
So who's the only being as vicious as Sion, makes Boba Fett crap his pants and just wants to kill?
DURGE!
http://orig03.deviantart.net/264d/f/2013/155/9/8/durge_demotivator_by_jerard_kal-d67ulbp.png
From drowning people in Lava
https://www.youtube.com/watch?v=s-a1muUm6F4
Laughing after being impaled by a lightsaber then beating the person who impaled him like the rented mule of a red headed step child
and face tanking a volley of missiles
http://img1.wikia.nocookie.net/__cb20060623004942/pt.starwars/images/a/a4/Gen_'daiface.jpg
I don't think that there is anyone else out there like Sion or Durge and giving Sion a Durge skin would be amazing. For more insight of the Gen'dai Mandalore Killing, unstoppable bounty hunter without any reading check this out.
https://www.youtube.com/watch?v=wmJqgdaTArM
If anything this would be a high price skin because of how much it would change for the Big Man.
Voice- a slight metallic filter would do IMO.
Sprinting animation could be Durge using his jetpack.
http://img4.wikia.nocookie.net/__cb20150712021622/starwars/images/2/2d/DurgeOnQueyta-Rep53.jpg
Axe/Melee Object
As shown in the video Durge uses a spear while riding aboard his speeder bike. That or his flail would make for a nice melee object. or he could just use the speeder bike he grabbed and used as a weapon.
http://www.tradingcarddb.com/Images/Cards/Non-Sport/82456/82456-5864235Fr.jpg
Passive-Like how Sion is unstoppable Durge is as well. When he's taken so much damage that his armor is ltierally blown off of him he goes into his pure form and relentlessly attacks his foes.
Q- Personally I would prefer if this part of the skin would have Durge Sion sing is flail back like in the show and bring it down with alot of force. Of course with the lesser version being a swipe and a slow.
W-Not really much of a shield right? But the Big Man does wear alot of heavy armor... Either way it's something nice to go off of, giving him massive gauntlet shields.
http://images.wikia.com/starwars/images/9/9a/Durge_using_his_energy_shields.PNG
E-While the normal Sion just roars, I think this should get Omega Squad Teemo's Q treatment. Instead of roaring, Durge Sion fires his flame thrower, flame throwing a minion forward/ slowing enemy champions.
http://moosemix.com/maxwell/CLONE_WARS_PAGE/images/obiwan_vs_durge2.jpg
Ultimate-This is easy for the whole Durge POV. Put him on his speeder with lance forward charging into his foes.
http://img3.wikia.nocookie.net/__cb20101130000342/starwars/images/8/8d/Durgeanddroids.PNG
and finally death animation. I know I know, how did someone like Durge even die? Well...stars exist.
http://static.comicvine.com/uploads/original/11118/111186862/4074343-2139200-new_picture__96_.jpg
While he did technically survives being thrown into a star he was only cloned later by the empire, not the real deal.
Just an idea with loads of images representing every possible thing about him. Sorry if there is too much clutter.