Might get some stuff wrong, but I remember hearing Kid say he stole something from Big Mom, this hasn't been touched on at all not even when Law and Kid were fighting Big Mom for two chapters, so I think what Kid stole from Big Mom was her logue book. At the end of Wano, Kid will give that book to Law and that's how he will discover what the will of D is, then he will fuck off and go back to his hometown to try to rebuild it.
We get some cover stories from his adventures and never see him again :)
We get some cover stories from his adventures and never see him again :)