Disclaimer: I don't own Homestuck or Rise of the Guardians. They their owners.
When the trolls had created their universe, it had prepared a place for them as the zodiac. However, Jack Noir had interfered and the trolls had not been integrated into the new world.
It was different for the humans.
When they created the new world there was a place for them as well, and this time they were able to fill their roles.
Jade, the Witch of Space, had always been cheerful and energetic. These traits were carried on to her new role, Toothiana the Tooth Fairy. Her home, once in a Jungle, remained in one and turned into the Tooth Palace. She traveled all over the world, making sure that no child's memories would be corrupted as hers had once been under the Condesce. Her own memories from her life as Jade remained intact, and she treasured them.
Jake, the Page of Hope, carried that trait onto his new role. He became E. Aster Bunnymund, Guardian of Hope. Once his home had been on an island, and he was drawn to an island once again. Easter Island was his own personal paradise and he spent the first century of his life hiding there, still remembering how conflicted he had felt while with his friends. When he finally left, Toothiana found him and restored his memories of being Jake English. He had been astonished, but guarded his past life carefully.
Dirk, the Prince of Heart, had possessed a great talent with robotics. This was carried onto his new role as Nicholas St. North, also known as Santa Clause. He had subconsciously chosen to learn from his past life and change, possibly thanks to one of his splinters. He made toys for children, just as he had made robots for his friends in another life. When Toothiana found him, he accepted his past memories easily and promised to fix his mistakes.
Rose, the Seer of Light, had looked to the Horrorterrors for guidance. She had been a Seer, and was able to choose the best outcome for herself. Through this she had chosen to protect others from the beings of the night in her next life as Sanderson Mansnoozie. She had been a Seer, and was able to choose the best outcome for herself. After all, her gender had never been that important to her in her first life. She guided the children from their dreams, as she had once guided her friends. When Toothiana had returned her-now his-memories, he had silently applauded his past self for her ingenuity.
John, the Heir of Breath, had been a mischievous prankster in his past life. He carried that trait into his new role as Jack Frost. The wind was his companion once again and they roamed the world together, having fun and playing with children that couldn't see him with his new powers of snow. When Toothiana met him she had been distracted with the fight with Lord English, now Pitch Black, and had forgotten to return his memories. She still treated him as the brother he had been in their past lives. When they were revealed to be missing he had chased them, and he got them back from Pitch Black. They were reunited and defeated their old enemy.
Three of their human friends were still missing, so the Guardians swore among themselves to protect the children and wait for their return. Dave, Jane, and Roxy were yet to be found in their new universe.
So here you go! The Homestuck kids as Guardians! Partially inspired by DomesticHeart's From The Stars, which you should definitely read. Feel free to use this as an AU/Prompt, just let me know so I can read the fic! I won't be continuing this.
A few notes on this universe:
Rose: She would have been Agender, but never told her mom and thus was always treated as a girl. She didn't care much about her gender either way, so she never really cared how she was seen.
Dave: I think he would be Nightlight from the Guardians of Childhood series. However, I kept this fic with the movie and thus didn't include him. Keeping with this, Calliope would have become Pitch's Daughter. It's been a while since I've read the books though, and I only read the first three books. If you haven't read them, I would suggest doing so. I also considered him for Sandy, but I thought Rose fit better.
Jake: He even has the ridiculous accent! I was initially unsure about him being Bunny, but as I wrote I found the parallels.
Guardians: Once they were reborn, they went fully into their new roles. They also changed to fit them, as shown with Dirk. They use their new names most of the time.