Easiest prediction is Nika and Yami
But Ope Ope fruit could be the case as well.
Imu could be straight up immortal, you maybe able to counter the regen by slowing down heavily if you got Joyboy lvl+ haki, but still won't able to kill.
This maybe because Ope Ope immortality was used by Imu in the past, and somehow could revert it back using Ope Ope again.
Law's having 100% involvement in the future, and I don't think Immortality surgery will be done to anyone, immortality=evil in my book so the purposeful way to use the last trump card of the fruit will be against the Big bad boss to revert back the Immortality.....thereby Law will sacrifice, indeed it will be a great sacrifice and one of the best way to send off a popular fan favourite.