Sasuke from Naruto could be a great addition. His brooding nature and powerful ninja skills would contrast well with Musubi's more upbeat and energetic personality. Also, Konohamaru could be included. He looks up to Naruto and his presence could add some humor to the story as he tries to impress Naruto and Musubi.
In the Musubi side, we could include some of her friends or allies. For example, Karasuba. Her strong and somewhat wild nature would create an interesting dynamic with Naruto. And from Naruto's world, Kakashi could be included. His wisdom and experience as a ninja would be very useful in guiding Naruto and Musubi when they face difficult situations.
From Naruto's side, Sakura could be a good character to include. Her medical ninjutsu skills could come in handy when Naruto or Musubi get injured. On the Musubi side, Hibiki could be added. His more laid - back personality would be a good counterbalance to Naruto's determination and Musubi's enthusiasm. They could all form a team to take on various challenges in the fanfiction.