If you wanted to have him say “Grandson”, I wonder if you could take him saying “Grand Inquisitor” from the Kenobi series, and him saying “Son” from the end of Empire Strikes Back when he is calling out to Luke.

You could also amalgamate the existing “Find me…” line, but use Vader saying “me…” (maybe taken from “join me…”), so it is a mix between Palpatine and Vader’s voice.

You could do something similar with the line “Fulfill your destiny” and let it be a combo of Palpatine (and/or Snoke) and Vader’s voices, since they all say some variation of that line. “Fulfill/it is your destiny.” Or just let Vader’s line play out as-is when he gets to the Wayfinder.