Why the Post-Legacy Era Antagonist Has to be Plagueis
Plagueis may lose to Sidious in ROTS or DE, but some things should be noted about this Sith:
Sidious might not have found a way to defeat Plagueis by strength of arms. As per the STDP novel, Plagueis' life force was stronger than Sidious', as the apprenticed sensed after assassinating his master.
Second, Plagueis was a mathematical killing machine, far more so than Sidious. His reaction times, his senses, were all better, he could kill like no other. He was a far more brutal master as well, he avoided unnecessary battles and would not spare his apprentice should he attack the master as Sidious did with Maul. The Magister was not as forgiving as the Emperor.
My point is that he discovered a mild version of Darth Krayt's dark transfer, but had a far broader application. He may not have been able to heal himself after death (as far as we know); but he could enhance his genetic fortitude, making him smarter, more lethal, and able to draw more upon the dark side. His midi-chlorians were increasing by his will, he was able to cause an immaculate conception and because of all this he would have advanced exponentially in his powers had Sidious not taken him out so soon.
Sidious had grown quite a bit stronger by Ep. III due his battle with Windu, and having his own power turned back on him - but if Plagueis were to come back from death, especially with a technique like mc manipulation, he'd automatically catapult to DE Sidious or maybe even FoTJ Luke levels. Allana may have the greatest Force potential in the mythos (being descended from a genetically enhanced line of Allya's and the line of Force prodigies stemming from Anakin Skywalker); and may become the first Jedi Queen (>Grand Jedi Master) - but if Plagueis had continued to permanently grow in physical power, faster and faster, he would eventually topple Nihilus, Vitiate, Gethzerion, and maybe even Abeloth the Bringer of Chaos in physical might. He would make a worthy adversary to Allana in post-Legacy events, wiping out the weak, One Sith impostors who rule under a Jedi Queen - having returned from his grave on Muunilist using his mc manipulation.
Plus, we have Cade who's just retired from the Jedi Order, one dabbling in the exact same field as Plagueis but from a more esoteric viewpoint. We know the Skywalkers are easily corrupted to begin with, and are the most powerful Force wielders in the galaxy, followed closely by the descendants of Allya.