The power of Nika resides within a DF. This DF is immortal and if you kill the user, the DF will just reincarnate until a new Nika emerges in the future.
The world exists in a pattern of cycles. Every time Nika appears the world is destroyed and the cycle begins anew. This time is the final cycle as this is the time Nika will finally win.
Hence in order to increase the stakes, it is very likely Oda will introduce a way to permanently kill DF souls. Either by the yami yami no mi, Imu, or some form of technology.
In any case, if Oda introduces a way for DFs to die, this would not only make it to where Nika can permanently die. It could also make it to where other immortal powers can also die. The yami yami no mi, the gorosei, and Imu himself.
Ultimately, a DF killer would be the weapon that ends the cycle. My guess is Vegapunk or Franky could create it. But Teach/Imu are equally likely to create such a weapon.
The world exists in a pattern of cycles. Every time Nika appears the world is destroyed and the cycle begins anew. This time is the final cycle as this is the time Nika will finally win.
Hence in order to increase the stakes, it is very likely Oda will introduce a way to permanently kill DF souls. Either by the yami yami no mi, Imu, or some form of technology.
In any case, if Oda introduces a way for DFs to die, this would not only make it to where Nika can permanently die. It could also make it to where other immortal powers can also die. The yami yami no mi, the gorosei, and Imu himself.
Ultimately, a DF killer would be the weapon that ends the cycle. My guess is Vegapunk or Franky could create it. But Teach/Imu are equally likely to create such a weapon.