The third dark lord

[Finwe]

Tolkien did mention that the Blue Wizards might have started cults in the East, influenced by Saruman, that could eventually cause trouble for the western regions of Middle-earth. Their fates are purposefully kept ambiguous, and it is expressly stated that Gandalf was the only wizard to succeed in his mission (which was to inspire others to rise against Sauron), so I'm assuming that means that our dear Ithryn Luin fell "into shadow."
As for the Elf topic, I wouldn't put it past some Elves. Just look at Maeglin. Some may use the excuse that the combination of his traumatic childhood and his capture of Morgoth drove him to madness. I still say that it was his CHOICE. Every evil person starts out good. Even Morgoth was "good" at one point. They just become evil through wanting to do their idea of "good deeds" and what is "good" for the world. Any Elf who was alive before the Dominion of Men could still believe that Arda was intended for the Firstborn only. Elves had remarkable persuasive capabilities. It wouldn't be hard for an Elf to scrounge up a good-sized following of Men to start "taking over the world."
To all those who suggest that a Gondorian king could follow the same path as Ar-Pharazon and Numenor, I say, read the Silmarillion again! Sauron was the one who persuaded Ar-Pharazon to leave him alive, and take him to Numenor as a hostage. After that, he slowly insinuated himself into Ar-Pharazon's good graces. That is how the Fall of Numenor happened. I don't think that anyone with any of Sauron's capabilities ever arose again, and only one of Sauron's persuasive skills could ever hope to persuade a Gondorian king to put him in his councils. It just would be stupid.