I just wanted to emphasize that Ahab, being neither criminal nor outcast, had no need for a "beyond" plan; with the death of Moby Dick, he'd be done. Lack of concern for the future would not bite back at him.
Khan in turn did have a pressing need for such a plan, as all possible outcomes of his original revenge spree would leave him a fugitive. With the death of Kirk, he'd be facing challenges greater than ever before.
Then again, Khan had shown competence at improvising when his original plans of killing Kirk fell apart; he quickly came up with the option of leaving Kirk marooned inside Regula and just destroying his starship. We might argue that, despite his debilitating obsession, he was still capable of coming up with plans very quickly, and would survive the completion of his revenge and start planning for continuing survival. And after pulling off that
, he'd be hungry for more, and would indeed proceed with conquest of the universe or whatnot.
Madness might mean Khan had no plan originally, but it would not
mean there would be no "what if" when he finally defeated Kirk.