I've bought this up before but the possible deaths of Chad and Rukia have inclined me to dedicate a new thread to it. KT explicitly stated that souls with high spirit density and high reiatsu are the ones who become shinigami. When their human bodies die, they end up in SS (unless they become hollows), where things like hunger cause them to be noticed and if they're strong enough they can join the Gotei 13.
Now that being said, as Chad, Ishida, Tatsuki, Yuzu, Karin, and Orhime are all living human beings - wouldn't it make the most sense that when they died they become shinigamis? I think theres a major plot hole with the death of Ishida's grandfather as in Soul Society he should not have retained his Quincy powers, unless of course... yeah. Assuming its not and giving myself some mental latitude, if the six of them could retain their human abilities as dead souls and still become shinigami, wouldn't that make for quite the creative and logical power up once they've reached the limits of their human abilities?
Let us not forget that its still entirely possible for Urahara to force them to become shinigami in the same way he did with Ichigo.
I dunno, I think this is quite an exploitable situation for the mangaka if he chooses to use it. Nothing would make me break out laughing harder than Chad's ban-kai. Although the possibility of Yoruichi training Tatsuki (an already gifted martial artist) makes me drool...