Sure, we can talk about the Quincy thing over PM but the idea is very simple. Quincy had this cool weapon, we wiped them out and then the science division made a crappy replica from observation and recovering the busted up chunks we could find, because having a portable sword can be nice and practical and a solid backup for any shinigami. They wouldn't be widespread for obvious cultural reasons, the attachment of a shingami to their zanpakuto would make accepting such a weapon slow and complicated.

I'm specifically trying to avoid pulling a Kenpachi because he's cool and already established, I'd rather try my own take on a similar character. Kenpachi is naturally ridiculously powerful, he can hand captain level fighters their ass without release and by holding back to boot. Meanwhile the idea behind my guy is that if he underperforms he's probably dead because he just isn't that gifted with reiryoku and reiatsu, rather than faceroll on power alone he has to survive on skill.

He'd always walking a razor's edge, since reiatsu also essentially translates to stamina and defensive power. Rather than simply overpower abilities as Kenpachi would do, he has trained and developed for hundreds of years to avoid the type of situation where he'd be rendered completely useless and he still carries plenty of weaknesses and limitations inherent to him. The entire point is to avoid Bleach's obsession with powerlevels and aura size more or less defining who wins and who doesn't by having a character who has, through literal centuries, worked his one specialized skill to a level where it can at least partially compensate and pull him forward in life and put him into a station he would not have attained by birth alone.