You said it with the interesting shinigami not being shown enough. They have characters they could have made good side characters but they didn't do it.
I agree, the problem with Bleach is how they choose to use their side characters. Naruto often neglects characters, to the point where some are only brought out to kill them off, but Bleach suffers from using their side characters almost entirely as a prop for Ichigo.
Someone like Ishida could definitely have gotten some more focus, but everytime he's brought out he's probably getting his ass kicked. Inevitably he has to defer to Ichigo. Orihime has a ridiculously powerful ability, but as a character she's never allowed to do anything more than heal or act hysterical. Chad probably had potential but he's also never developed. Renji arguably suffers it the worst, he capped out in about the second arc and has spent the rest of the manga being a DBZ-esque villian prop-up character. By defeating Renji, we know the villain is strong! Characters like Yoichi, etc., definitely have the backstory and the power to compete, but they're operated almost entirely in the background, simply pushing Ichigo further when he needs something that they could provide rather than being a meaningful part of the series.
I think it'd be hard to say that Naruto out and out has better supporting characters, I mean we're supposed to feel sad about the death of a character you might have seen in all of a half dozen frames occasionally. But at the same time they avoid turning the character into just a prop for Naruto or Sasuke. They might die an ignoble death after a couple of appearances (including most of their backstory dumped on the manga right before the kick it) but their progression and development wasn't at least entirely subordinate to the main character(s).
