Skywalker is the Resistance
While the events of Star Wars: The Last Jedi led to most of the Resistance getting wiped out by the First Order, the ending of the film hinted at a new generation of children who support the Resistance spread throughout the galaxy. In an interview with Variety, Abrams said that "some time has passed" between The Last Jedi and The Rise of Skywalker, so it's possible that the kids we saw at the end of the previous film could be young Resistance fighters in the next one. "Skywalker" could be the name they adopt for themselves, especially once word gets out about how Luke Skywalker faced down the entire First Order by himself.
While the previous theory still kept the focus on Rey as the first one to take up the Skywalker mantle, this one shifts that focus to the Resistance as a whole. Having a group collectively refer to themselves as Skywalker, named for a single man who stood his ground against overwhelming odds, seems to be in line with Abrams' inclusive interpretation of the Star Wars mythology, and would provide a satisfying note for the end of the saga: if we choose to stand for what is good, even in the face of great evil, we are all Skywalker.