My idea for the plot:
Once upon a time Pitch appeared in the north mountain and started preparing to take over Arendelle and then all over the world.
Next day Jack arrived at Arendelle and by accident met Elsa during snowball fight. Elsa likes Jack so she introduced him to Anna.
Next day pitch attacked Arendelle, Jack and Elsa fight him but Pitch nearly killed Jack and kidnapped Elsa. After that Jack woke up at city centre and felt sorry, then decided to take revenge on pitch and save Elsa.
Jack met Anna and Kristoff and they started to look out for Elsa.
After a long journey they seem to found Pitch but they found only dark Marshmallow created by Pitch, they decided to split up, Anna and Kristoff distracted dark Marshmallow`s attention from Jack to let him continue looking out for Elsa.
Some day later he finally found Pitch`s hideout, released Elsa (after romantic song;) )when Pitch was busy taking over the Arendelle.
Arendelle became dark so Elsa and Jack flew back quickly and had epic final battle with Pitch, they defeated Pitch and
Some time later Elsa and Jack got married.
There are so many possibilities! If Disney and Dreamworks would work together (which is very unlikely), I think they would make a great movie. Anyways, here's my idea:
During the time when Elsa isolated herself, Jack was flying once and he saw her in her room. He saw her sitting in front of the door, her back facing the door, and there was ice everywhere. He was curious so he came in but he didn't know that she could see him. After that, he started visiting her every day and they became best friends. Then they fell in love but none of them admitted their feelings to each other. During the events of Frozen, Jack had to leave for his guardian duties and when he came back he saw a new Elsa. They talked again, but then they found out that Pitch was back and he was trying to send nightmares to Elsa so he can collect her fear. Then Jack, Elsa, Anna and Kristoff go to Pitch's hideout underground and then while Anna and Kristoff are dealing with Pitch's nightmares, Jack and Elsa have an epic battle with Pitch and they defeat him. After that, they go back to the castle and Jack and Elsa finally admit their feelings to each other. Then the movie can end with them kissing and the whole kingdom celebrating.
Elsa and Anna are already united once more, but not for long. As a matter of time, Anna saw a black guy which she thought was the slenderman (what? XD).Pitch tried to kill Anna, but Elsa saved her, which killed Elsa instead. When Anna saw this, her heart of love disappeared. She was so stressed that she lost her sister, she became so mad. As she became Queen of Arendelle, love can't control her any longer because of losing Elsa. Instead, Pitch did. Pitch made Anna evil. The next day, Kristoff noticed Anna's hair turn black and her eyes were yellow. Anna is already evil. Everyone in Arendelle was so surprised about their new Queen. Meanwhile, Elsa, was standing there, wondering who she was. Jack came near her and took her hand. Elsa gasped. Jack and Elsa went to the North Pole. Many days have passed, Jack and Elsa became closer and fell in love. Jack proposed to Elsa and Elsa said yes. "I love you, Elsa" Jack said. "I love you too, Jack" Elsa said, but wait! do they know that Arendelle is in trouble? Do they know that Anna has became evil? "Bad news" North said. "What is it?" Jack and Elsa said. "Pitch is attacking Arendelle, Elsa's home" North said. Elsa's eyes widened. "HOME?!" She asked very shocked. Tooth gave Elsa her memories. Elsa found out. "Anna! Now I remember" Elsa said..
Anna became more evil as well. She earned powers, which is fire. Elsa and Jack came back to Arendelle. "Anna?" Elsa said. Anna could see Jack, but not Elsa. "are you Jack Frost?!" Anna said evilly. "Yes, and I'm with your sister, you have to believe in her". Anna tried to control and she saw Elsa. "Anna?" Elsa said. "Elsa..." said Anna. "It's me" Elsa said. Elsa and Anna spoke, Anna became good once again! Her eyes and hair turned back to it's original color. Elsa, Anna and Jack fought Pitch. Pitch was defeated. Anna told everyone in Arendelle to believe in Elsa. Jack and Elsa got married and so did Kristoff and Anna.