Would it be unrealistic for Satsuki to be the reincarnation of Saber?
Considering Saber never died (not only did she not die in her legend, she can't truly die when summoned as a Servant, because killing her as a Servant just sends her back to her slumber in Avalon until she's summoned again)... yes, yes it would be both unrealistic and just plain weird for Satsuki to somehow be her reincarnation. This isn't even getting into the absolutely massive differences between both their respective worlds.
If it's any comfort, I imagine the two of them would get along fairly well.
