Many speculate this means that Luke defects to the dark side, or at least struggles with the temptation to do so.
However, there is also a less sinister explanation. Some argue that the layout of the poster positions each character against their primary nemesis. Rey aligns with Kylo. Finn with Captain Phasma. Luke with himself. Perhaps this means that Luke’s greatest foe will be himself, that the demons from his past — his regret over being unable to prevent Kylo’s fall into the dark side or the mass slaughter of his fellow Jedi — will be his greatest obstacle.