Yo!! Here is the first idea, well, technically, it has been sitting in my head for a long, and I mean, a long time!  This chapter would be for my Kamen Rider book, if you haven't read it, go ahead and check it out!

This chapter would be placed right in the middle of the Final War Arc.

Chapter Starts:

As the battlefield grows more chaotic, as All for One leads the charge against the heroes, on another corner, tensions are high as the Kamen Riders are busy fighting Aku, the dark Kamen Rider.

Aku: What? You think that you can stop me? Even in your final forms you cannot even lay your hands on me!

He declared as he was in his Ryuga Survive Mode, towering down the other Kamen Riders.

Jack, Senkai, and Kado struggling to get up as the damage has been done.

Jack: We can't give up... Y/N... is counting on us...

He said as Y/N was nowhere in sight, the effects of his last battles against Shigaraki and Aku took its toll as he was in a coma.

Kado: We can't do anything... He has gotten too powerful...

Senkai: That's a lot of crap!!

He said as he staggered to stand up his body was in bad shape, all of them are, but Senkai, he wasn't ready to give up.

Senkai: What would he do... If he sees us... Like this...

He stood up, glaring daggers at Aku.

On another part of the battlefield, as the heroes band together to stop the villains from attacking, this involves the UA, Y/N's school as well.

As Eraserhead was near Monoma who uses his quirk to repel Shigaraki's quirk, on another end, Bakugo suffered heavy casualties as Monoma tried to stop himself shedding a tear, trying to support the heroes.

Y/N: Hey, hey... Stop that noise... That really ticks me off...

Monoma and Eraserhead jolt a bit as they heard a familiar voice through their comms (assuming that they have) as they look for him.

Eraserhead: Y-Y/N...

Y/N: Don't act surprised now sensei. You told us to come back safe yeah?

Eraserhead was happy, no, ecstatic to say the least, but he doesn't want to show it in public, but nonetheless, he was happy to hear the familiar voice of 1-A's one and only Kamen Rider.

Y/N: Monoma, keep up the good work, and how's about you make a portal, for me?

He asked as Monoma chuckled, he just smiled for a moment and did what Y/N asked.

Monoma: Give them hell.

Back with the Kamen Riders...

Senkai: Did you forget about what we promised... We will fight for everything, for peace, and for our friends!

He yelled as the other two tried to stand their ground, injured and battered as they looked forward to Aku who was staring down at them.

Aku: Utterly pathetic. In this world...

He said as Dragblacker stood beside him as two figures stood beside him...

Uscias: It's kill...

A figure who was holding a draconic spell book emerged as it was Uscias (The Guardian).

Volcanus (Villain Story OC): Or be killed.

The other figure said as he was wearing a modified black and yellow hazmat suit that is ready for battle. Tubes stemming from his gauntlet connected to his body was seen as well.

Aku: End of the road, this is your final stand. This is where. You. All. DIE!!!

He said as Dragblacker and Uscias, were charging up for an attack, while Volcanus' arms started to glow, channeling the very magma of his body towards his gauntlet.

Decade Driver: Attack Ride! Blast!

Bullets started to fly across the field as Dragblacker was shot, roaring in pain as Aku looked at the direction that was shot as well as the injured Kamen Riders.

Walking out of the dust was the H/C haired teen, wearing his hero costume, bandages across his body as he wasn't 100%, but he was ready to fight.

Aku: Impossible! I left you for dead!

Y/N: True, but the fire and determination that I have can never be beaten. I have people counting on me, and it would be a damn shame if I let them down!

Y/N said as he glared daggers at Aku, as Aku laughed at his counterpart.

Aku: How noble. In case you forgot, your friends are useless, they are badly hurt, you think you can stop us? You're even more pathetic than I thought!

Y/N: Who said I came alone?

He said as if right on cue, a white background, similar to Decade's antagonist power, Narutaki, emerged as Y/N was right, he didn't came alone.

Out of the screen, a figure who was holding a scythe emerged, dawning his guardian attire, the Bloodstone around his neck, and a sky blue streak of aura on his right hand.

Guardian Y/N (The Guardian): Well, he wasn't wrong, it felt it was a right call to even up the odds.

He said as another figure walked out of the screen before it disappeared. Wearing a formal suit and coat that looked like much of a mafia boss, a gas face mask, and a cannister around his waist.

Ryuu (OC): We won't sit idly by just to see his future would go to waste.

Ryuu said as Y/N smiled and looked at his Kamen Rider friends, the one who stood up for him and the ones who had believed in him since day one.

Y/N: Thank you, for believing in me. Zi-O!

He said as the Ziku Driver appeared on his waist as a golden ridewatch appeared as he pressed it, signaling to his friends to get out of the battlefield as it will get messy. He pressed the sides of the watch as Guardian Y/N and Ryuu walked a few feet apart.

Ridewatches: Zi-O! Grand Zi-O!

He placed both of them on the Ziku Driver as coins started to drop down and silhouttes in gold of the Kamen Riders appeared behind him.

Ziku Driver: *Kuuga chime*, *Agito chime*, Advent! Complete! Turn up! *Hibiki chime*, Change Beetle!, Sword Form!, Wake up!, Kamen Ride!, Cyclone! Joker!, Taka! Tora! Batta!, Three! Two! One!, Shabadoobie Touch to Henshin!, Soiya!, Drive!, Kaigan!, Level up!, Best Match!, GRAND TIME!!!

Y/N: One last fight! Henshin!

He said as he turned the Ziku Driver and proceeded to transform.

Ziku Driver: Kuuga! Agito! Ryuki! Faiz! Blade! Hibiki! Kabuto! Den-O! Kiva! Decade! W! Oz! Fourze! Wizard! Gaim! Drive! Ghost! Ex-Aid! Build! IWAE! Kamen Rider, Grand Zi-O!

As he finishes transforming, he is clad in gold armor as figures of all past Heisei Kamen Riders are at his disposal, proving that he came a long way with everyone that he loves and cares about.

Y/N: (Guardian) Y/N, Ryuu, let's finish this.

Guardian Y/N: I'm at your disposal.

He said as he placed his scythe on the back of his shoulder (kinda like Seele).

Ryuu only nodded as ashes started to form around him, levitating him off the ground as ashen wings took form.

Aku, Uscias, and Volcanion started to get ready, rage, and violent intentions starting to form as Y/N looked back behind him.

Y/N: All of my life, I was alone, I felt nothing matters.

He said as figures of his friends standing with a determined look on their faces, past Kamen Riders, and most important of all, his classmates, Eri, and the teachers of UA came into view as with a determined look on his face, he was ready.

Y/N: But now, I have one reason why I kept fighting. I fight... for our future!

He said as the 3 opposing parties started to charge each other, signalling the start and end of their personal wars.

Chapter Ends

Well, what do you think? Also, for the OCs, yes I have Ryuu my BNHA OC filled in the spot for the meantime. I have somewhat an origin story on Wattpad from before but it was unpublished and forgotten hehe.

Anyways, here's one idea and I hope you liked it so far. Feel free to express your ideas or opinions about this. Please take note, this is a what if, so it may or may not happen. Who knows?

Anyways see you on the next one!