What will happen if the Marvel Universe collides with the DC Universe? Well, let's find out. Note: This is based on my opinion. This is how I imagined a Marvel vs DC Royal Rumble fight. I did this to join a Marvel vs DC Contest so....yeah, I hope you'll like it.
In the Marvel Universe, the Illuminati (Iron Man, Mr. Fantastic, Beast, Black Bolt, Black Panther, Dr. Strange, Captain Britain, Amadeus Cho and Hank Pym) is still on a look out if any Incursions will occur. J.A.R.V.I.S, then, detected an incoming force headed towards earth. When the Illuminati checked it out, it was another earth. Thinking that they can stop the other earth from colliding with their own, the team traveled to the other earth to find a way to destroy it.
In the outer atmosphere of the DC's Earth, the watchtower detected the Earth-616 and with this, the Justice League (Superman, Batman, Wonder Woman, Flash, Aquaman, Green Lantern, Cyborg, Shazam, and Green Arrow) went and looked for a way to eliminate it. The two ships that carried each teams had met in space with each team declaring the destruction of the earth opposite from theirs. A battle between the Illuminati and the JL took place on the Blue Area of Earth-616's moon. The battle resulted with the humiliating defeat of the Illuminati but the Justice League, even though they won, admitted that the their opponents were not easy to take down. The Illuminati retreated and regrouped back in Earth-616 while the JL, confident that they can take down any threats that will get in their way. The JL went back to the watchtower to think of a way to destroy the other earth.
Back in Earth-616, the Illuminati shared the information that they got with the Avengers, Fantastic Four, and the X-men. Captain America, thinking that this might be the greatest threat that they will ever face, brought out a call to every hero available in their world. The Marvel Heroes were surprised when the Justice League arrived within their atmosphere. Both sides engaged in a destructive battle. Most of the Justice League were occupied by their opponents but Superman was able to beat the heroes that got in his way. Superman was about to deliver massive punches to the ground in order to destroy the planet but he was hit by Mjolnir. As the hammer flew back, Superman looked up to the sky and saw that it was Thor who threw the hammer. Angered by the fact that Superman was about to destroy Earth-616, Thor flew as fast as he can and engaged into combat with the Man of Steel. The battle was intense with the fact that every blow that they deliver, the ground beneath them shatters.
While the other heroes are locked in combat, Iron Man, Mr. Fantastic, and Hank Pym traveled to the other earth to find a way to stop the Justice League and possibly destroy that earth as well. When they arrived in Metropolis, they were greeted by Lex Luthor, who offered them help to take down the JL. While he was talking, Iron Man scanned news and history about that world. Stark found out that Lex Luthor is a villain of that world and denied his help but as they are about to leave, they received a message saying that they are getting beat up by the JL. Tony had no other choice but to accept Luthor's help. Lex handed them a piece of kryptonite and tells them that it will help them solve their problem by bringing down Superman. Before the three heroes left that planet, Luthor asked them a favor: destroy the JL.
Back in the battlefield, Batman was locked in battle with Captain America. Both heroes are equal in terms of skills but Wayne admitted that Rogers has the upperhand in terms of strength. The Punisher, who is in possession of a bullet that is made out of a Vibranium-Adamantium Alloy which was given to him by Tony Stark and Black Panther, aims a sniper at Batman. Seeing this, the rest of the Bat Family (Red Hood, Nightwing, Damian Wayne, Catwoman, Tim Drake, and Bat Girl) ambushed the Punisher. Realizing that he is outnumbered, Frank sent out a call to the Thunderbolts (Red Hulk, Deadpool, Agent Venom, Ghost Rider, and Elektra). The Thunderbolts got the upperhand when the Red Hood was locked in the Penance Stare. Elektra got her hand busy with both Catwoman and Batgirl but their fight ended with both Elektra's sais thrusted in the bodies of her opponents. The two Robins didn't stand a chance with the power of the symbiote when Agent Venom lost control and murdered them. Finally, Nightwing almost defeated Red Hulk but the Punisher shot him 3 times which enables Rulk to finish the battle with devastating blows and a snap in the head. Punisher carried on in firing the specialized bullet to Batman. The bullet was able to break into Bruce Wayne's battle armor and it nearly kills him.
With the sight of this, Green Lantern summons the Green Lantern Corps. which really turned the battle in favor of the DC Heroes. The Lanterns almost decimated a large portion of the Marvel Heroes. Captain America ordered S.H.I.E.L.D. to call backup which arrived in the form of the Nova Corps. led by the Guardians of the Galaxy and Richard Rider. The Nova Corps. took the battle in space in order to distract the Lanterns and to keep them away from the battle in Earth.
When Iron Man, Mr. Fantastic, and Hank Pym returned to Earth-616, they went to the Baxter Building to try and create a weapon using the kryptonite. The three heroes thought that Thor would give them time to work but luck was not by their side and so is time. In the battlefield, the battle within the Thunder God and Superman came to a halt when Thor was teleported back in Asgard. The Marvel Heroes lost their only hope when the Odinson left earth. Captain America commanded every hero of Earth-616 to turn their attention to Superman but the Man of Steel was aided by the other JL members.
The Flash took on Quicksilver but since the battle takes place on another earth of another dimension, Flash was slower than he normally is. With the absence of the Speed Force, Quicksilver was able to match Flash in his own game. The Son of Magneto was able to distract Flash long enough for Dr. Strange to open a portal to the Dark Dimension. With Flash out of the way, the battle became easier for the Marvel Heroes. When Aquaman saw that the battle was turned against them, he summoned all of Atlantis' forces to help them in the battlefield. With the absence of Namor, the heroes of 616 cannot ask help from their own version of Atlanteans.
With the help of DC's Atlanteans, the Marvel heroes got distracted and in the long run, their fall was inevitable. Most of the X-men and a large portion of the other heroes had fallen. But a game changer came to earth's atmosphere. The Hulk, who seeks revenge to those who blew up Sakaar and to those who shot him in space, was angered by the fact that the DC Heroes attacked his targets. He wanted to be the one to take down the Marvel Heroes. This caused him to unite with the other Marvel Heroes to kill the DC Heroes. He quickly took out Shazam with blows that has the potential to destroy a planet. After that, he turned his attention to Superman. The Man of Steel faced Hulk with everything he's got.
Meanwhile, Iron Man, with the help of Mr. Fantastic and Hank Pym, had finished building a suit of armor that is powered with the only weakness of Superman. With this, Iron Man summons all of his armors and commanded them to attack the Atlanteans and help the other heroes. Stark flew to the battlefield and helped Hulk. He fired a Unibeam directly to Superman which initially weakened him. Hulk charged to Superman, temporarily knocking him down.
With the help of the Hulk, the Marvel Heroes easily took down the remaining Justice League members. But the Man of Steel woke up and flew near the sun, absorbing energy from it. When he finally absorbed enough energy, he came back to earth and decimated most of the Marvel Heroes. The only ones who remained standing was Captain America, Spider-man, Iron Man, Hulk, and Wolverine. Iron Man tried to hit him again with a Kryptonite-powered Unibeam but Superman was able to shrug it off. The Man of Steel flew to Iron Man and ripped apart the armor. With the anger seen in his eyes, Superman was ready to end the battle.
Spider-man, who was nervous, asked Captain America why he's not freaking out. Captain America answered him by saying, "Because I hear thunder". Lightning went down to the ground and hit Superman. When he looked up, he, again, saw Thor with a new battle armor. This is because when he was teleported back to Asgard, he learned that Odin had let him use the Odin Force to aid him in the massive battle against the Justice League. As we all know, the Odin Force is the most powerful magic in all of existence and on the other hand, Superman is weak against magic. A devastating battle between Superman and Odin Force Thor took place.
Odin Force Thor was able to control the course of the battle and got the upperhand against Superman. But when the Thunder God was about to strike the final blow, Superman used his new power: The Solar Flare or the Super Flare. He was able to knock down Thor and every hero that is near them. Of course, this attack also knocked Superman himself and turned him into a human. After a few hours, Superman and the other heroes woke up. Now that he's human, Clark was surrounded by the remaining Marvel Heroes with Spider-man giving the final punch to the "Man of Steel". The Marvel Heroes, with the help of the Nova Corps and the warriors of Asgard that Thor summoned, was able to knock down the Lanterns and the Atlanteans.
The Marvel heroes brought the DC heroes back to their earth. Without them knowing, Batman, injured because of the bullet shot by the Punisher, had already woke up. He used every gadget that he had and every skill that he knew to take on the Marvel Heroes but Wolverine, with his healing factor, ruthlessly killed the Dark Knight. When the heroes returned to their respective earths, Mr. Fantastic built a machine that can support the Ultimate Nullifier and used it to eliminate from existence the earth of the DC heroes.