While Luke Skywalker may have ended the original trilogy as a true Jedi, there were moments in his life where he was tempted by the way of the Sith. Darth Vader initially intended to turn Luke to the dark side and usurp Palpatine as a new Sith duo, while Palpatine intended Luke's slaying of Vader to turn him from the light side. In Star Wars Legends continuity, Luke even became the cloned Palpatine's dark side apprentice before ultimately turning against him. However, despite these brushes with darkness, Luke never got his own 'Darth' title.
A mark of immense power and supremacy, the 'Darth' title has a long history in Star Wars lore, stretching back to before the Rule of Two, which decreed that only two Sith - a master and an apprentice - could exist at one time. Originally a symbol of status within the Sith order - one which challenged other adherents by proclaiming the bearer's supremacy - it became more standardized after the Rule of Two for new Sith Lords to adopt the title, which was used by major figures such as Darth Sidious, Darth Vader, Darth Maul, and Darth Tyrannus.
While imposing, these Sith titles don't tend to be particularly sophisticated, jettisoning subtletly in favor of pure dread. Vader and Sidious simply remove 'in' from two menacing words, while Darth Ruin and Darth Bane don't even go that far. Darth Azard and Darth Kallous make small spelling changes to negative words, and George Lucas even famously suggested the names "Darth Icky" and "Darth Insanius" for Star Wars: The Force Unleashed's Starkiller. It's perhaps because of this rejection of subtlely that Luke Skywalker's childhood nickname of 'Wormie' offers up an ideal Sith monicker (albeit with a little retooling.)
Luke's Wormie nickname was originally depicted in a deleted scene from Star Wars: A New Hope, and entered canon thanks to 2015's Star Wars #5 (from Jason Aaron and John Cassaday.) While a little goofy, it serves to emphasize Luke's humble origins and the lack of satisfaction offered by his life on Tatooine. Of course, 'Darth Wormie' and even 'Darth Worm' aren't exactly intimidating, but it doesn't take much to turn them into something that is. 'Darth Wyrm' would fit with Sith tradition, and its original meaning - an archaic term for 'dragon' - would properly convey Luke's incredible power, and even has some poetic meaning as the inversion of the Jedi 'knight' he could otherwise have been.
The Sith's names are often handed out by their masters, but they also commune with the dark side in order to make the right choice. As Sith are empowered by fury and hate, repurposing a warrior's mocking childhood nickname would likely only add fuel to the fire of their power, especially since this would forever tie Luke to the life he did everything possible to leave behind. Star Wars has engaged with alternate timeline stories in the past - even presenting a story where Darth Vader returned to the Jedi - so it's possible that fans may get to see a Sith Luke in Marvel's comic stories sometime soon. Until then however, Luke Skywalker's mocking childhood nickname offers up Darth Wyrm as a surprisingly appropriate identity for the young hero should he have fallen to the Sith.