it is a good story with a fresh new idea of an insert character, with a nice way to implement in new forms the powers of already in story devil fruits, another set of powers that you could implement for the yomi fruit would be the powers of danny phantom because they are a underworld type of racial abillities that the ghost in that universe all have, ectoplasm attacks, elemental attacks, banshee wails or shout, etc. other types of underworld powers would be instead of ice it could be fire, like in god of war games, helheim is a cold place of the dead but in grece the tartarus is a helish fire place. other type could be necromantic powers, just like original momonga, calling bones from the fallen and using them for spears(bone lance), walls(wall of skelletons), maybe a skelleton hands or ribs to grab an enemy or something, other cool things to add for possible power ups could be the techniques of the rinnegan or the samsara eye, meaning eyes of the reincarnation or such things, those powers are actually even more possible with the combiantion of the yami fruit and his gravity manipulation, having acces to the shinra tensei, maybe revival of other things or persons via inner energy or something external, even a dnd call of the dead, to speak with dead people to get their techniques as well, many possible powers to little space to write
