Why couldn't Sidious uncover immortality?
Why could he not do what Plageuis and the Sith Emperor of the Old Republic Era could do.
It is apparent that the Sith Emperor was a genius of all aspects of the force, his 1,400 years of his Sith research, Alchemy and Sith knowledge was immensely superior to Traya's, and somehow Sidious and his billion worlds could not get his hands on the Sith Emperor or Plageuis' discoveries, why?
Perhaps the Sith Emperor and Plageuis have some sort of connection, and either of them may have been an even more terrible Dark Lord of The Sith than the already awesome terror that was Sidious. Plageuis and the Sith Emperor might have been the same being, whom might have caused the Force's imbalance and retained its own spirit inside of Sidious after dying, which was destroyed with Sidious' flesh during his first death aboard the Death Star II, causing the imbalance to be fixed.
The Plageuis Novel was canceled indefinitely, I suspect because of the Old Republic, Darth Revan, The Exile and the Sith Emperor's plot must unfold before the Novel can be made due to a connection between Plageuis, Sidious and the Sith Emperor.