Toon Wars: Return of Finn

A rogue clone of Finn the Human and his fanbase seek to bring Adventure Time’s popularity and glory back from Steven Universe by attacking it and ending the series. To prevent them from succeeding, Peridot and Nicole Watterson (whose world had already been attacked by Finn's army) summon the help of the Nicktoons (Spongebob Squarepants, Danny Phantom, Jimmy Neutron, and Jenny Wakeman). However, by the time they arrive, the fanbase manage to reach Steven’s earth. Each of the heroes must now battle against the forces of Adventure Time in order to save the world of Steven Universe and Gumball from cancellation. This is a sequel to The Toon Wars and How to train your Hero.

Characters

 * Spongebob Squarepants: Since the Toon Wars, he along with the other Nicktoons have fought in many different events, including their last crossover with How to train your Dragon. Because the original Finn was the one who helped him become who he is now, it is a priority to Spongebob to defeat his rogue copy in honor of his name.
 * Finn the Clone: This copy of Finn was created by the fanbase of Adventure Time after the Toon Wars with the sole purpose of regaining their cult following and keeping others from surpassing it. He despises Steven Universe the most out of all cartoons because he had managed to go beyond Adventure Time's standards and create a much larger fanbase. Desperate, he believes the only way to bring back their popularity is to destroy any other world that rivals their own.
 * Jake the Dog: Among the fanbase, Jake had helped to bring his best friend Finn back to life. However, when he discovered that it was indeed a clone of him and not the real person, Jake had left the army. He is the one who contacted the Crystal Gems to warn them of the attack.
 * Nicole Watterson: After the Toon Wars, Nicole's son Gumball returned to the show with his family. Next to Steven Universe, TAWOG grew a large and dedicated fanbase. But when the Adventure Time fan base attacked the show in an attempt to destroy it, it was Nicole who drove them away. To keep them from returning and destroying Steven Universe, she helps Peridot and the Nicktoons defeat the Adventure Time army.
 * Danny Phantom: A popular character with a large cult following who helped the heroes in both the Toon Wars and the events of How to train your Hero. He is summoned by Jimmy Neutron to work with his friends to defeat Finn's army and invited Jenny Wakeman to assist them.
 * Jimmy Neutron: He is a genius who once served in the Toon Wars and was called by Peridot to get help. He is the one who contacted Danny and Spongebob, but didn't tell Timmy Turner and Invader Zim about the attack believing it to be a dangerous task for them.
 * Jenny Wakeman: An Android with a soul surviving many wars and drama as the iconic teenage robot. She is brought by Danny to help fight against Finn's army.
 * Peridot: Peridot was told by Garnet and Nicole that the world was soon going to be attacked by Adventure Time. She is responsible for contacting the Nicktoons and being a part of the battle against Finn's fanbase.
 * Toothless: Previously the main character of How to train your Dragon and the savior of the cartoon universe during How to train your Hero, Toothless had married Comet the Pony and now resides on an unknown planet with her. He is seen training Spongebob in the intro of the story.

Plot
The remainder of the Adventure Time fanbase has revived the original Finn the Human after the events of the Toon Wars. This Finn is the one from the earlier seasons of the show before the events of Lady and Peebles. Afterward, a replacement was made in his honor. These fans reveal to the original Finn that he himself is also a clone, but is made to be exactly like the real one. Yet, the clone, after seeing that Adventure Time is slowly dying down because of other shows like TAWOG, gets upset and seeks to get the show’s glory back by force. He and his fanbase attack Gumball's world as a test to get their strength back, and later set their sights on Steven Universe for revenge, since Nicole was more than a match for them. Meanwhile, Jake the Dog, angry at Finn for attacking the innocent, decides to leave the fanbase and warn Steven Universe about the impending attack.

 Back at Steven's world, Garnet is informed about the invasion and Peridot overhears the conversation. She believes she can help the Gems by gathering more heroes capable of taking on Finn's army, but Pearl and Amethyst believe they can finish the job by themselves. Steven and Garnet, however, tell Peridot to do whatever she can to help, and Garnet suggests she calls Jimmy Neutron to get his friends to assist her. Peridot calls Jimmy and tells him about the attack, and he quickly gets into contact with Spongebob, who was training with Toothless on their new home planet. Toothless asks if he can help, and he and Spongebob come up with an idea. Later, Jimmy calls Danny Phantom for help and he gets Jenny Wakeman to go with him for assistance. Meanwhile, Peridot calls Nicole to ask if she wants to help them take on the fanbase, to which she heavily agrees.

 A day later, the heroes arrive early at Steven's world, however, all but Peridot are still asleep and far away from the battlefield. Jimmy says though, that they detected the fanbase coming to the planet at a rapid pace. Suddenly, they appear and Copy Finn shows his face to them, making everyone upset that the real Finn is still deceased. Spongebob vows to save Steven Universe in honor of Finn's name, and Copy Finn orders his men to attack! The heroes separate and attack their own individual waves of enemies before returning to fight copies of Finn's friends. Eventually, the battle dies down and Copy Finn is nearly defeated by Spongebob, but when the Crystal Gems finally arrive onto the battlefield, Finn activates a backup weapon: An Unused Brainiac Super Laser from the events of HTTYH. He sets it to fire directly at the planet to destroy it for good and teleports inside of it to command the rest of his military. The Gems start to panic, but Garnet and Spongebob start laughing. Garnet says she can see the future and Spongebob confirms that he also has a backup plan. He states that Toothless knew of a much larger fanbase, and Comet was in charge of it. Known as My Little Pony, the many beings from the cartoon had already occupied the super laser and ambushed Finn and his army. Outnumbered, Copy Finn surrenders and Toothless contacts Spongebob to tell him that the plan was a success.

 After the battle, Copy Finn is taken into custody by Danny and sent into the Ghost Zone for twenty years. A party is held at Steven's home town and Garnet tells Peridot that she is proud of her for taking the initiative despite the Gems saying they could do it own their own. Comet calls it teamwork and Nicole's family arrives to praise her for her actions in the story. Spongebob speaks to Steven and happily discusses the times he had with his friends before and after the Toon Wars as the story finally closes.