
Darth Plagueis was Palpatine's Sith Master before Palps killed him in his sleep. TLDR, this is the Sith lord Palpatine told Anakin about during RotS. possibly because Palpatine learned it from him. His vision of the Sith matches what Palpatine ended up going for almost exactly. He seems to have pissed off the "true Sith" something fierce (note how the Sith spirits on Korriban don't seem to support him, via omission). He's supposedly been alive for a thousand years or more.
Jossed: Official novels make it clear that Revan left a pregnant Bastilla behind, and never had further children before being trapped by the Sith Emperor. The idea is the Exile fell for the new Revan that resulted from the events of KOTOR (canonically light-side) and they became a couple when she joined him in fighting the True Sith after the second KOTOR game. She was put in charge of the action where he sent all the dissenters to get killed. This would contradict the implication in KOTOR 2 that the exile was opposed to Revan's fall, though. And isn't surprising parentage a classic Star Wars plot point? Even the non-Force Sensitive classes could be latently gifted and unknowingly calling on the Force to be the best of the best. If this is the case each class would have their own backstory as to how the descendant of two powerful Jedi ended up with the Sith, as a smuggler or a bounty hunter, etc. Two former comrades in arms fighting a hopeless battle together.
Satele Shan is already suspected to be a descendant of Revan and Bastila, but there may be other reasons that the canonical genders of the two KOTOR protagonists are different.
Mind you, it's worth pointing out that this WMG actually has supporters within the game: the head of the Revanite Order on Dromund Kaas believes it, and Imperial PCs can choose whether they believe it or not. but then the Republic frees him, and the restraint in the Emperor's head is now gone. He was "absorbed" into the Emperor's mind, which is why he was showing such restraint when dealing with the Republic. And because he felt sorry for the Sith, he started a war that killed billions of innocent people? Doesn’t sound like LS Revan. At this point, his body has mostly deteriorated (allowing Bioware to avoid canonizing his appearance) only through the power of the Force has he been able to survive this long. Deciding that the people living in the Empire deserved to get their homeland back, he took the Emperor's place in order to both reform the Imperial government from within and recapture the ancient Sith territories from the Republic. On the way, though, he learned more about the history of the Sith and began to sympathize with them. Revan, having arrived in the Sith Empire, confronted the thousand-year-old Sith Emperor and killed him.