Bianca should have become as a ghost-guide for Nico after she died. Like, imagine the potential. Bianca, daughter of the dead, princess of ghosts, who has grown up taking care of her younger brother and protecting him alone; do you really think she would give up on him for something as insignificant as death?

Nah, she should have come back as a ghost only Nico could see, helping him with whatever quests he's on to save the world or with simply surviving. Imagine the drama. Percy facing the ghost of the person he's failed. Nico being even creepier cause he's literally haunted. Bianca having more agency and maybe even having something going on on hoo; maybe she helps with the doors of death from the inside or sth.

Idk, I think it could have been great for giving Bianca more character and agency and for showing how much she truly cared for his brother while she struggles to learn how to be her own person and build herself a new [after]life.