Pearl, Garnet and Amethyst would surely be main characters. They were probably waiting anxiously for Steven's return. Once he is back, they could help him readjust to life at home. They might also assist him in dealing with any post - traumatic stress or new challenges that emerged from the Grem war.
Maybe Steven comes back with new powers. He could have learned some unique combat skills during the Grem war. For example, he might be able to control a new type of energy that he discovered while fighting the Grem.
Besides Steven, the main characters could include the villains he's faced in the past. Now that he's a girl, they might underestimate him or treat him differently. His father Greg could also play a big role. Greg might be a bit shocked at first but then would be super supportive, trying to understand what his 'new' child is going through. And there could be new characters introduced, like other girls in the town who become friends with Steven and help him adapt to the female world.
Another theme is growth. After going through the war, Steven is no longer the same innocent boy. Fanfictions usually explore how he has matured, both in his understanding of the Gem world and his place in it, as well as in his relationships with others. He might become more assertive or more empathetic.
Sakura could be a main character. She might be the one who helps Naruto deal with his emotional and physical wounds after the war. She could also be struggling with her own feelings towards Naruto now that he's back.
Ichigo and Steven would be obvious choices. Ichigo is the main protagonist in Bleach, strong - willed and with great power. Steven, on the other hand, is kind - hearted and has a unique way of looking at the world. Their different personalities could lead to some interesting interactions. They could start off as strangers in a new combined universe and gradually become friends as they face common enemies or challenges.
If it was a crossover with "She - Ra and the Princesses of Power", Adora could be a main character. Her strong leadership and the power of She - Ra would be an interesting contrast to Steven's more laid - back leadership style. Together, they could fight against a common enemy that threatens both their worlds.
Garnet could be a great addition. Her wisdom and calm nature could provide some contrast to Steven and Amethyst's more impulsive personalities. She could also offer some valuable advice to them during their adventures.
One popular 'Steven return from gems war' fanfiction might be 'Steven's New Beginning'. In this story, Steven comes back with newfound wisdom and strength. He uses his experiences in the war to bring about positive changes in Beach City, mending relationships and starting new initiatives for peace among the Gems.
It could be a new villain that has emerged. This villain might have some sort of vendetta against Steven specifically, perhaps because of his unique powers or his role in the gem world.
Well, clearly Steven and Connie are the main characters. They are the focus of the fanfiction.