What if: Luke joined the Dark Side?

What if Luke joined his father in The Empire Strikes Back and it was up to Leia to save the galaxy? How would Return of the Jedi have played out? I have no idea so I'm interested in hearing other people's thoughts.

Discuss.

I'm just guessing, but I imagine they would have ruled the galaxy as father and son.

I know at least one person on this site would respond by saying, "that's an impossibility. The canon has already been set. Sephiroth is the only possible candidate to become..."

Oh... wait...

Anyway, there has been one novel where Luke turned to the dark side. Although this was post-RotJ. I think Luke would gradually become more and more like his father: eventually becoming consumed by hunger for power and paranoia that his father is still only manipulating him. Eventually, he would kill him and take on an apprentice of his own. Mara Jade, Jacen Solo, and Zekk are all candidates, but Jacen would most likely be the one of primary interest.

Leia would abandon her blaster for a lightsaber, but it's likely that she would suffer many losses, particularly the death of Han Solo as Luke would begin to torment her in order to inspire her to anger enough to turn to the dark side herself. This would also be the catalyst that would drive Jacen to the dark side, wanting enough power to take revenge on his uncle for his father's death, and his sister Jaina would be driven to do the same but while yet following her mother's path.

Luke would probably fail to fully turn Mara Jade and she would keep his son, Ben, from him.