Alright I wrote this while I was bored out of my mind, and this is my first fight, so keep that in mind.
On one of the final islands in the New World, two great superpowers prepare to clash....
(At top of ruined castle)
Blackbeard: Straw Hat! I knew you would come! The battle between the two contenders for Pirate King! This is the battle of my era!
Luffy: Blackbeard, there can only be one Pirate King. And that's gonna be me.
Blackbeard: Zehahahaha! Such determination! Well then come at me Straw Hat! Show me the famous power of yours, and let me crush it like I did to your brother and old man Whi...
(Luffy appears in front of Blackbeard covered in smoke)
Luffy: Jet Bazooka!!
(Blackbeard is hit hard in the stomach and sent flying back into the wall)
Blackbeard: Argh!!!
Luffy: Gear Third!
(Luffy inflates his arm and pulls in back preparing to strike. Blackbeard gets up from the rubble and looks up to see Luffy's giant fist)
Blackbeard: Eh?
Luffy: Gigant Rifle!!!!
(The entire top of the castle is blown off and Blackbeard is sent flying out above the ruins that surround the castle. Luffy jumps out as well.)
Luffy: Gomu Gomu....
Blackbeard: (Holds out his hand) Kurouzu!!
(Luffy pulls back his inflated arm and puts his feet together.)
Luffy: Spear!!
(Blackbeard is caught by surprise and is shot to the ground.)
Luffy: Now for a....
(Blackbeard looks up and grins.)
Luffy: Gigant Rifle!!
(The attack hits Blackbeard, but as the smoke clears, Blackbeard is seen standing in front of Luffy and holding his giant fist off with one hand)
Luffy: Eh?
(Blackbeard swings back his fist. Luffy gets a sudden premonition of the attack and rapidly deflates his arm, using the air from Gear Third to blow himself out of the way)
Blackbeard: Quake Fist!!!
(The island rumbles, and what is left of the castle is split in two and falls to pieces)
Luffy: This is just like old man Whitebeard! How did he get that power.......?
(The smoke clears and their is a gigantic fissure right where they had been fighting. Luffy stands up out of the rubble and looks around for Blackbeard, seeing him standing at the edge of the fissure.)
Luffy: You bastard!!!
(Luffy goes to charge, but he realizes he cannot move. Blackbeard looks at him and grins.)
Blackbeard: Black Hole!!!
Luffy: Crap! (Tries to run but can't, so he pulls back his arm instead) Gomu-Gomu....
Blackbeard: Quake Fist!!!
Luffy: Pistol!!!
(Blackbeard's earthquake hits before Luffy's attack can reach. The quake expands the fissure and splits the entire island in two. Luffy loses his balance and tumbles into the crack)
Luffy: Gomu Gomu no...
(Luffy grabs the edge of the fissure with both hands. Blackbeard envelops one hand with darkness and the other with a quake bubble)
Luffy: Rocket!!
(Luffy shoots straight at Blackbeard, but Blackbeard raises his darkness hand)
Blackbeard: Kurouzu!!
(Luffy is dragged to one side, and Blackbeard grabs Luffy with his darkness hand, forcing him to the ground)
Luffy: Color of Armanents....
Blackbeard: Quake Bubble!
(Blackbeard punches Luffy in the chest with his quake fist. The ground cracks and splits around Luffy, and he coughs blood)
Luffy: (cough) Gomu Gomu no Stamp!! (Kicks up from ground and hits Blackbeard in the chin)
Blackbeard: Quake...(Raises fist again)
Luffy: Gomu Gomu no Jet Ono!!!!!
(Blackbeard's head is smashed into the ground by the attack. Luffy switches the Gear Second to his arms)
Luffy: Jet Gatling!!!
(Blackbeard is hit many times by the onslaught and is thrown backwards a long distance. Luffy charges after him in Gear Second)
Blackbeard: Black Hole!!!
(Darkness spreads across the entire island rapidly and envelops the surrounding forest and the ruins. Luffy, noticing the attack, starts jumping off of the ruins to avoid being sucked in)
Luffy: Jet Twin Pistol!!!
(Blackbeard is hit but holds his ground. The darkness sucks in the island's terrain, reducing it to a wasteland. Blackbeard holds out his hand)
Luffy: Jet Rifle!!!
Blackbeard: Liberation!!!!
(Luffy's attack hits Blackbeard straight in the face, but the island's contents are shot at Luffy. Luffy manages to deflect the debris, but Blackbeard is waiting for him)
Blackbeard: Quake Bubble!!!
(Luffy is hit hard and coughs blood again. Blackbeard pins him to the ground with one hand covered with darkness.)
Blackbeard: Liberation!!!!!
(More debris is shot straight out of Blackbeard's hand holding Luffy, dealing heavy damage. Blackbeard raises his other hand)
Blackbeard: Quake Bubble!!!
(Luffy is slammed in the face point-blank. Blackbeard raises his limp body and throws it into the air, then rears back with his quake hand again)
Blackbeard: Quake Fist!!!
(The air shatters directly in front of Luffy. Luffy spits blood, but the force of the attack sends him flying a long distance.)
Blackbeard: Zehahahahahaha!!! Not even the famous Strawhat can face off against my Gura Gura no Mi and Yami Yami no Mi!!!