WMG: One Punch Man

Saitama's power is reversing cause and effect on attack and defense.
Like Lancer's Gae Bolg from Fate/stay night, he does something; and reality warps so that the effect is that his punch crushes people, or their attacks on him aren't as effective; regardless of his physical strength. Similar to Lancer; the ability has an Equivalent Exchange on his luck. Unlike Lancer, he doesn't get hurt; however people never recognize what he does. (Because they can't rationalize the shift in cause and effect.)

Saitama is a product of the same phenomenon as the random mysterious beings that are mutated human beings.
The series has introduced examples of human beings that have repetitively performed certain tasks that cause them to transform in a fitting fashion. A man who ate too many crabs became Crabrante, a giant crab man. A person shadowboxed a ceiling light pull cord so much that he became Personification of a Light Pull Cord, a powerful and fast boxing giant. These beings were still people, as they spoke normally and the first example even engaged in a somewhat civil conversation with Saitama at first passing. They were influenced by their power, however. One sought revenge for being drawn upon, and the second desired to test his strength against objects and people.

While these beings were people influenced by their transformations to the extent that they became disruptive to the lives of others, Saitama is different from them only in that he is heroic and doesn't desire to harm innocent people with his power. His extremely powerful punch and generally superhuman fitness is a showing of his desire for strength that he sought through training. He trained so much and so hard that whatever phenomenon caused the changes in the other two beings mentioned happened to him as well. As the others transformed into monsters, Saitama transformed as well; he simply became bald. Thus, he is literally a monster fighting other monsters, but he is a monster that retains most of his original human form.

Speed of Sound Sonic and Flashy Flash are related.

Both of them have redundant names, boast their superior speed and use the Wind Blade Kick as well as sharing extremely similar combat styles. And both reacted exactly the same when meeting Saitama for the first time.

One-Punch Man takes place in the future of Mob Psycho 100.
Mob psycho despite officially being part of the same world, lacks all the mysterious beings and there are no heroes, and psychics and ghosts are only slowly becoming a thing (also a young Tatsumaki and Fubuki are shown at one point). Most likely something happened between Mob Psycho and One-Punch Man that led to a mass empowering, allowing people to push the limits of humanity or go over the edge and become monsters. Mob Psycho likely takes place around 8-10 years before One Punch Man, and BLAST is widely believed to be a grown up Mob (who would be around the same age as Tatsumaki, + or - a couple years).

Saitama will eventually find a challenging enemy... who will beat the living shit out of him because ''he's'' too strong.
Because honestly, the irony would be too great to see.
By the time Genos finds the cyborg that destroyed his village, he will vastly outclass him
Think about it: Genos has been shown to ask Stench to improve him every time they meet up. Genos has also be shown to fight on even terms with city destroying monsters (e.g. Sea King), and power level far below that would be enough to destroy a village. Therefore, it seems likely that by the time Genos finds the mad cyborg, he will be far above him in terms of strength.

The series will end with Saitama fighting God
I see it playing out similar Asura's Wrath: God reveals that he's responasible for much of the Earth's suffering (or maybe is just too lazy to do anything about it), Saitama gets pissed, they fight (with Saitama finnally fighting at 100%) and Saitama either A) killing God, B) dying in the attempt, or C) both.

Metal Knight and the Hero Association are secretly part of an organization creating the Mysterious Beings
Metal Knight is a jerkass from the start as he uses a situation of a city-destroying asteroid to test out some new weapons without actually saving anyone. His actions later on include avoiding combat, looting alien technologies, and building a super-fortress for the Hero Association. That also makes the Hero Association suspect as without the Monsters there would be no need for Heroes or for people paying the Heroes. What do they need with a Super-fortress anyways? There is already evidence that the Hero Association is supplanting city organizations like the police with Heroes. the Hero Association itself seems full of weak and/or jerkass people with a few strong and/or real heroes so they're nearly useless against any threat unless Saitama saves them.

King actually does have a superpower: His luck
He may not be the most badass superhero alive, but he's been in so many situations that could've splatted him, and yet managed to survive from several timely factors, and even gotten a terrifying reputation he can save his own life with more than once, it all seems to be a little too much for a coincidence. So perhaps something within him is organizing these coincidences, letting him leech off others' fame for himself. He and his luck don't seem to agree too much on what they want, but it'd seem that's his actual power.

This world is the future (or past) of Soul Eater
Just like the Kishin, madness and obsession can gift people with Lovecraftian superpowers. The only difference is that heroes have relatively benign obsessions while Mysterious Beings succumb to the madness inherent in theirs. In the more powerful heroes like Tatsumaki, Metal Knight and Fubuki we start seeing the genesis of Witches. Saitama might be a precursor to (or actually IS) Shinigami, someone who has effectively become a Humanoid Abomination en route to ascending to an Anthropomorphic Personification of Death.

Related to the above theory: The Hero Association is going to become a dictatorship.
Let's see. The H.A. is going out of its way to make the police look like fools and useless fodder. The Heroes are slowly becoming the first and last line of defense with no military units or government organizations in sight. Metal Knight is producing bigger and badder weapons "for peace". They built a superfortess on the ruins of a devastated city that no civilian is allowed access to instead of making it a super bunker for resident civilians. Metal Knight has a walker ready made to make the fortress in 7 days as if he KNEW that the city would be destroyed. The H.A. seems more concerned with public appearances and naming superheroes instead of protecting the people. The great prophet conveniently dies right before giving the details to the upcoming catastrophe causing confusion and panic while under the direct watch of the H.A. who should have been monitoring her the entire time. Mysterious Beings have started popping up everywhere with seemingly no explanation and people with super powers are inexplicably popping up everywhere as well. Plus Drive Knight tells Genos that Metal Knight is his enemy. All that tells me that something fishy is going on.