Star Wars: The Last Jedi saw Kylo Ren / Ben Solo choose the path of the dark side in pretty decisive fashion, by killing his master, Snoke, and usurping control of the entire First Order. However, the Star Wars saga is pretty circular when it comes to the fall and redemption of the Skywalker family, and director Rian Johnson is already teasing that Episode IX still has room for Kylo to find his way back to the light.
When speaking to IGN, Johnson outright stated that Kylo Ren can be redeemed, pointing out that on the scale of Skywalker evil deeds, Kylo isn't nearly as bad as his predecessor:
"Are you kidding? Vader was worse than Kylo ever was, I think, and Vader got redeemed. Also, I should just for the record [say] that I’m not involved in the writing of the next movie. I’m an audience member in it, just like you, so when I talk about what’s going to happen next it’s in the context of, as a fan, what I’m thinking of."
Star Wars Episode IX has bigger dangling threads to tie off than The Last Jedi ever did. One of the biggest is redefining the nature and conflict of light and dark side Force users, now that it seems like ideas of The Sith and Jedi are being retired. There's still room for Kylo Ren to become something like a Gray Jedi, balanced between light and darkness - and there's certainly room (and a certain amount of expectation, at this point) that he also plays out Vader's arc in full.
Kylo Ren ultimately coming back to the light as Ben Solo would be in keeping with this Sequel Trilogy's echo of the Original Trilogy - which seems to be the idea, here. How that redemption happens, and how it sets up the future of the franchise remains to be seen, but since Kylo Ren is pretty much the most interesting character in the ST, it's certainly a possible storyline fans will be buzzing about.
Star Wars: The Last Jedi is now in theaters.