I'll get straight to the point. Aoi and Kokuto didn't deserve their respective fates. Both of their deaths (including Kinko) progressed the story, to make it clear that nobody was safe. However, the difference between Aoi and Kokuto's deaths compared to Kinko's is that of how their deaths (more so Aoi's in my opinion) were used for shock value. Kokuto was killed by a booby-trap after finding a set of keys. Kokuto for the breif time we have with her in the anime, light novel, and manga is seen as confident, smart, and reasonable. Her death, while setting stage for the fact no one is safe in the death games, goes against her character. Aoi's death is EXTREMELY brutal. She is killed, not because of her own mistake, but by Kinko cutting her own hand off to secure the key. Aoi's death doesn't foward the plot, while Kinko is overwhelmed with guilt, Aoi's death just shows how brutal the game is. Kinkos death was needed, as it progressed the story and showed that Yuki, while kind and respectful, she isn't above killing if it means she is to survive. Kinkos death also fowards the story with her father, and how he wants the games to end.

So, how would they bring Aoi and Kokuto back? Well, it could be through flashbacks, of how they were introduced to the game OR they could bring up how, the characters who are in the games, are seen as dolls. Instead of blood, its white stuffing. Dolls can be put back together. 

(please just bring them back, I like their personalities, designs, and they seem to be wasted potential)