With the time travel of Legends of Tomorrow, the alternate realities of The Flash, and the flashbacks of Arrow, there may always be a place for Malcolm Merlyn and John Barrowman.
Of course, none of this is to say that Malcolm Merlyn could absolutely never return to the Arrow-verse. Unlike a certain Supernatural star, John Barrowman doesn’t hint at any bad blood between himself and the producers, and all of The CW’s superhero shows have been known to bring back dead characters in new and unexpected ways.