Star Trek: The Original Series debuted a challenging foe for Captain Kirk in the episode "Space Seed." Ricardo Montalban played the part of the augmented Khan Noonien Singh who, like most villains, want world domination....or space domination. Of course, Khan was defeated by Kirk and sent to Ceti Alpha V to live out the rest of his life with his team of soliders and new wife. But then the planet became a wasteland, which cost Khan his wife, and he and his team were forced to eke out an existence. We all know how that worked out when Khan returned in Star Trek II: The Wrath of Khan.
Khan dies at the end of the movie, but a spec script for Star Trek: The Next Generation had the idea to bring him into Captain Picard's world. Supposedly, according to writer and director Marc Bernardin, who worked as an intern early in the series. The Next Generation had the idea to bring Ricardo Montalban back for a visit. In an interview with Vulture in 2017 [via Slashfilm], Bernardin said Captain Picard would have been concerned about all the downtime the crew was having because of the long amounts of time it took to get from planet to planet. He wanted his team to stay sharp, and he shared his concerns with Data .
Everyone knows Data is a problem solver. So, the next thing the crew knows, the Enterprise is under attack by none other than Khan. He manages to cripple the ship so he could face off against Captain Picard. Then viewers would have had a chance to see how Picard handled Kirk's greatest enemy. At then end, though, it wouldn't have been an actual battle as it was a holodeck simulation that Data created to prevent the crew from getting stale during downtime. So while we would have seen Montalban return, it wouldn't have been the real Khan.