I can think of a few that I'd like.
Zanpakutō: Inevitability
Shikai - Dead end: At the mere touch of its blade the victim is flayed off his soul.
Bankai - Indefinite destiny: Allows me to alter the outcome of any event and even negate my own death.
Zanpakutō: Nexus
Shikai - Dimensional prison: The one who's struck by the blade is sealed in a universe without spatial or temporal dimension automatically nullifying the victim's existence.
Bankai - Spatial projection: Transports me to a universe of my creation removing me from all the potential harm of the battlefield. From there I'm able to create proxy projections, sharing all my skills and attributes, and partake in the battle. The death and demise of these proxies does in no way affect me.
Zanpakutō: First ray
Shikai - Corona: At release it sends out an omnidirectional wave of electromagnetic and/or fiery properties abrupting technological equipment and cremating nearby foes. The heat properties and the electromagnetic properties can be altered so that either no heat- or electromagnetic waves are emitted. After the release it makes me as fast as lightning.
Bankai - Achromatic fire: To whomever or whatever the edge of my blade is pointing at at the realize is set on fire. It's an invisible and inextinguishable fire that incinerates the soul of my victim. Past the release it makes me able to alter the material properties of my body and travel at light speed.
A good way to tell if something is overpowered is to compare it to the other Zanpakuto in the series. If yours absolutely crushes all opposition with narry a difficulty, it is overpowered. If it has it's limits and is not a good match up against some in the series, it probably is a good zanpakuto.
All of Astners failed to qualify sadly. Blax, what Division did you choose again? 😮
I could see a space-warping one work if it was limited.
Like,
Shikai: Bends the amount of space between you and your opponent, letting you slice them up while beyond their reach in return, and to avoid attacks by increasing the space and being further than you look (with the possible counter of good timing; hit when you attack and you'll be hit if you can't avoid it, or do an attack with range so great it'll hit you even with the warp effect).
Bankai: Extends the warping to flat-out teleportation. Seemly teleport between a few nearby locations with the real one in one spot and illusions in the other ones, but in reality be in a state of quantum-warp between all of them, so when the various images of you attack, if any blow is unblocked that one will always be the 'real' one, and the only way to be hit is to hit all of your locations simultaneously otherwise it'll pass through harmlessly.