(Well, not really new because that's official character, but the character himself has new stuff with him.)

There's a timeline where Arcturus Mengsk in order to return from dead someday, sold himself to Amon from Protoss Pantheon. How much? Well, Arcturus returned from dead of course, surprisingly as himself in his late 20s. Additionally he got supernatural powers of some sort. Then he overthrew his son, threw him to unidentified prison in unidentified location, and casted some powerful spell that somehow made everything look like nothing happened. Only heroes who already lived before previous events such as Amon's fall were unaffected. No Queen Of Blades being evil, no Brood War, nothing like those. Just new war started eventually in the end. Somewhere in the meanwhile Amon requested for Arcturus various secret tasks that would work as a decent payback. Little did Arcturus knew was, Amon wants to possess and then fuse with him when he completes all his tasks and goes to location later pointed by Amon. There Amon would have just wait for Acrturus to die so Amon would be reborn in his new powerful body of a fusion known as Arctur-Amon.

(Arcturus belongs to Blizzard)