One of the concepts for The Wrath of Khan pitted Admiral Kirk against his own son

Star Trek II: The Wrath of Khan brought Khan Noonien Singh [Ricardo Montalban] back as the villian he'd originated in Star Trek: The Original Series' "Space Seed." Khan still hated now Admiral Kirk {William Shatner] and wanted his revenge for being left behind on Ceti Alpha V, but other than having Khan as the villian, the one-story page concept Harve Bennett wrote wasn't similar at all.

According to The Fifty Year Mission The First 25 Years by Mark A. Altman and Edward Gross, the second Star Trek movie was originally titled "The War of the Generations." It had Admiral Kirk taking over command of the Enterprise to save a lost love from danger on a distant planet as she was being held captive by a band of rebels. When Kirk arrives, he, too, is taken captive, and he discovers that his son is the leader of the rebels. There's no love lost between the two men, and Kirk's son sentences Kirk to death for reasons unknown.

But there's something more nefarious going on behind the scenes as the true leader of the rebels is revealed to be Khan who wants to use a Federation starship, namely the Enterprise, to conquer the United Federation of Planets. Despite the volatile father/son dynamic happening between Kirk and, presumably David [he wasn't named], the two manage to begin working together. With their combined forces, they defeat Khan and fly off into the sunset together, so to speak.

The one thing I really like about this idea was it would have given Shatner and Montalban screen time together whereas The Wrath of Khan only had them sharing viewscreens. Alas, though, we didn't get to see this version, but it still has the makings for a good story, and the concept could work today, just with a different captain and a different villain.