The goal of darkening the storyline of Bleach is a tricky one. TBH, I don't think there is just one death to darken the story other than Yama-ji. Otherwise, a massacre would have to be performed to get the desired effect. If Yama-Ji dies, one of the Captains or perhaps a member of Zero Div would step in his place. But let's face it, Yama-Ji is not going down quietly. Orihime, hmmm.... Killing Orihime wouldn't exactly pull it off, because Ichigo has never shown his true feelings for her in the first place. We have no idea of how close they are to each other, and therefore, the fans are usually left wanting something more when it comes to this particular Manga. That lack of expression hurts Bleach in this regard. And since many fans themselves have stated numerous of times, that they didn't want the characters to show those kind of emotions, it would be difficult to explain how Inoue's death would cause Ichigo to go over the top other than she was a good friend and assume she meant something to him. What's that? She was by his side? So is Chad... It's the difference between me killing your dog and me killing your mother.