My idea for that scene, however controversial, would’ve involved ghost Anakin taking a holoprojector from the DSII wreckage and showing Ben a small recording of Padmé. Anakin would talk about his regrets and how he could never see her again, warning Ben profusely about his path. It would parallel Rey and Luke’s discussion, furthering the dyad theme.
If ghost Anakin is more involved in showing Ben the light (or has his own arc through the ST of helping the heroes), perhaps a reunion with ghost Padmé would be a good (albeit cliché) touch.
That could work very well, especially since the recording could easily be of a small moment from either the prequels, TCW or maybe something else and it would work because then depending on how they chose to deal with it and have Kylo either be redeemed or not, it could shake Ben to his core to hear so earnestly from his grandfather just how much this path took from him and if it doesn’t redeem him? Then it’d be emotional for Anakin as he’s watching as one of his descendants is not only going down the same route he is but he’s making even worse mistakes.