Fist of the North Star/WMG

Kenshiro's shirt is an extension of his aura.
Wouldn't that make sense? There's only one explanation for where he gets them from: they are just brightly colored manifestations of his energy, his to discard and recreate whenever he wishes. This is also supported by the fact that he only destroys them when he's very angry; obviously, losing control of his emotions causes the aura to be unstable and disintegrate.

Both Souther and Kaioh told their Monster Sob Story to Kenshiro to deliberately get a merciful and painless death.
Both men saw what Kenshiro can do to the human body, and both knew that our hero was the superior warrior. Telling him of their traumatic childhoods would therefore get enough sympathy from him to give them a merciful death... which they both got.

The nuclear war that made Kensiro's world the way it is is a direct result of Emiya Shirou's wish to be "The Greatest Hero Of All Time" in ''Fate: Unlimited Blade Works.
''

Think about it for a second: Emiya Shirou wished to be the Greatest Hero ever lived, and the Holy Grail, being the Jackass Genie that it is, interpreted it as: ''"I'll make the world into such a hellhole where you will be forced to be a Great Warrior just to survive. There, boy, now go and be that Great Hero."

Sure enough, the Holy Grail caused the leaders of our world to engulf it in nuclear fire, reverting it to a state of medievalism without firearms (thank Buddah) and where Warriors and Martial Artists have a place in the world once more (thank Buddah again). Kenshiro was only one of many Saviors that rose out of this world of pain and chaos (one man can't be everywhere on this vast desert planet at once, after all); Emiya Shirou, forged by the fires of endless battle, became the Savior known as Archer.
 * By extension, it's possible that Fate/Extra takes place after the destruction of the 'modern' world (as demonstrated through both Twice Peaceman's flashbacks and what we see of the world during the ending scenes) but before it descended completely into Mad Max-style pre-technological anarchy. This would indicate a timeline where the clashes between the rebels and the Harway-led plutocracy didn't stop with just germ warfare gone wrong but actually concluded with nuclear bombardment-- Possibly meaning that, across the radioactive sea, Vault-Tec and the former USA is compensating for a lack of Nanto Seiken practitioners with old-world tech and Pip-Boy-wielding Vault Dwellers/Chosen Ones/Lone Wanderers/Couriers. Who can, coincidentally, achieve similar results through use of VATS and the Bloody Mess perk. One timeline gave birth to two separate universes... Three if you think The Book of Eli takes place within the Fallout world, given the Great War/destruction by nukes and the solo sojourner defending human kindness/morality through the rampant, bloody murder of the obvious villains.
 * The reason the Fallout/The Book of Eli examples contain fewer Manly Tears / Tender Tears / Redemption Equals Death? Shirou's idealism, demonstrated in how he's able to forgive 'villains' like Rider and Sakura because of their tragic pasts, carried on through the heroes who managed to exist even in the Grail's Crapsack World. Kenshiro is a hero in the Shirou mold, as opposed to Archer who pursued his ideals even if it meant innocents dying. On the other hand, Fallout/Eli is a case of American Kirby Is Hardcore. Obviously, the reason for the difference between the martial-arts-filled East and the guns-and-tech West is a matter of local deity: As seen through Eli hauling that Bible cross-country thanks to the 'voice in his head' and the number of Biblical references in Fallout, notably the importance of a Biblical verse in Fallout3 and Joshua Graham/the New Caananites in Fallout New Vegas, the deity responsible for guiding/remaking the West - okay, or maybe just the US - isn't Buddha but God.

Kaioh deliberately made himself look like Raoh
Someone made a comic out of Kenshiro and Raoh's last battle and Kaioh read it. He liked it so much he wanted to reenact it for real, and what better way than to somehow get the real Kenshiro to head over to Shura for a revelation of his past to couple off with a battle between him and Kaioh?

Kenshiro summons new outfits through sheer awesome.
Given all the other crap he pulls off on a regular basis, it's at least plausible.

The man who carried The Book of Eli was to America After the End as Kenshiro was to Asia, and was a practitioner of Nanto Seiken
Even saviors like Kenshiro and Raoh can be restricted by geography and oceans, so the Gentleman Upstairs decides to give each area a savior of its own: and Eli has WAY more mastery in the art of slicing than a non-Nanto Seiken practitioner has any rights to.

Kenshiro can use Bullet Time.
Which is why his voice is so high-pitched when setting off a Hokuto Hyakuretsuken: he's firing each punch at what seems like normal speed to him punctuated with a loud "TA!". Normal humans see and hear only the sped-up version.
 * Alternatively, Kenshiro uses VATS and has the Bloody Mess perk.
 * Makes sense, in the fight against Devil he punches much slower, and instead of going ATATATATATA he goes ATA! ATA! ATA!

Saint Seiya is a prequel to Hokuto no Ken

 * After defeat Hades once for all, Athena wasn't need to come back to earth...she was wrong. Now, Lin is the new athena. Of course, Ken is the last Saint alive.

Someone Dangerously Genre Savvy with a Death Note is trying to track Kenshiro following the trail of bodies

 * Either that, or Hokuto's real secret technique consists in making every single idiotic road warrior thug in a hundred kilometers radius attack you. (Hokuto's secret mass aggro?)

Kenshiro (and/or Raoh, alternately) is one of the missing primarchs
Why not? The Primarchs are large manly men that are warriors beyond compare. The two biggest warriors beyond compare in Hokuto no Ken are Kenshirou and Raoh.

The nuclear war was indirectly caused by first contact with oni-shaped aliens
From this we know that not only Hokuto no Ken is set in the same world of Urusei Yatsura, but that Kenshiro frequented Tomobiki High in the same class as Ataru, Lum, Mendo and Shinobu (and that could explain Shinobu's super strenght: she probably saw Jagi or Raoh showing off and managed to copy the way Hokuto practitioners gets theirs), and was apparently the banchou. As the various aliens tended to leave around pieces of their technology, the various governments tried to get some of it, with the skirmishes progressively growing and degenerating in open war. Then the Oni moved in to recover Lum and her husband (and possibly some of their friend), the Americans mistook it for another invasion and fired a volley of nukes, China mistook it for the first strike and launched its nukes, and it went to full-blown nuclear war from there. _________________________________________________________________________________________________