Maybe Palpatine’s plan was to have Vader meet his clone on Exogol had Luke killed him on DS2.

Would be interesting to have Vader know that Palpatine won’t die on the death star, it would certainly explain Vader’s change of tune from ESB where he planned on overthrowing him to ROTJ where he’s completely submissive to his rule.

I could imagine an alternate scenario where after Luke strikes down the Emperor, Vader tells him that he is still alive on Exogol and that they must find a way to destroy him once and for all.