I think Shaisam was the pattern's replacement for the DO if Rand decided to kill Shaitan. I just finished another listen through of A Memory of Light and noticed that Shaisam was killed After Rand choose to let Shaitan live and just fix the bore. I think the pattern's purpose for Shaisam was a backup Dark One. Since Rand choose not to kill Shaitan and just repair the bore, the Pattern didn't need a replacement DO so Shaisam was no longer needed. If I remember correctly, Shaisam mentioned he wasnt reborn yet, he needed "a place to infest, a place where the barrier between worlds was thin." This is just a theory but I think if Rand did decide to kill the DO, Shaisam would have made his way into the cavern where he would have encountered The Dragon about to close the bore and after a confrontation, end up imprisoned in it and replacing the DO. Hell maybe in the next third age of this hypothetical scenario Shaitan will manifest itself like Shaisam did and end up getting killed by the "Matt" of that era because "Rand" decided not to kill the DO (Shaisam).