Star Wars: 8 Ways The Rise Of Skywalker Sets Up Episode 10
6. Rey Rebuilding The Jedi Order
At the end of the movie, Rey adopts the surname "Skywalker", and looks off into the sunset clutching her newly-crafted gold lightsaber. From a certain point of view, this is a perfect ending - evil defeated, galaxy at peace, true identity uncovered - but from another, this is the jumping off point for a whole new story.
Even with Palpatine gone (we think - he's been "gone" before, after all), the galaxy will always contain a few evil-doers and assorted sinister forces, and there will always be the need for a galactic police force or protective organisation of some kind, whose sole mission is to keep the peace. And that's where the Jedi come in.
From what we know, Rey is the only Jedi left in existence, and therefore, is the only one capable of building a Jedi Order, something to combat Episode X's inevitable evil threat. We know that other Force-sensitives exist - so Rey wouldn't be alone - and isn't it likely that she'd want to seek out others who may be struggling with their power, like she once was? That Skywalker branding will definitely help her rally people.
Plus... what is Rey going to do otherwise? Go back to Jakku and return to her life as a scavenger? That would be a waste of her abilities, and it only makes sense for her to pass on the strength of the Jedi to a new generation of aspiring galactic protectors.