First up the Rey costume description makes mention of something that could be spoilerish as it hasn't been confirmed yet
The mission to take down the First Order is underway! After invading the Starkiller Base, one member of the Resistance has split from her group and found herself in a massive room full of First Order soldiers. Completely surrounded by Stormtroopers with blasters and
Knights of Ren with lightsabers, she seems to be totally outnumbered.
They did confirm that Kylo Ren was a title for a group but never said that other members of that group would become involved in the story. Going by that description it seems Kylo won't be the only one showing up.
Now two things from Aftermath which I haven't finished yet so might be more to come of this stuff.
First off one of Palpatine's former advisors and Sith worshipper has a role in the book and makes mention of stories in the past that Palpatine was secretly sending Imperial forces out beyond the known galaxy to set up listening posts and bases out there. The advisor even advises his current allies that they should do the same as they are not in a position to deal with the New Republic.
All of that is very similar to what was done in the EU with Palpatine sending Thrawn back into the unknown regions of space to watch and prepare for the eventual return of the Yuuzhann Vong. Or just wishful thinking on my part because I want Thrawn in the movies.
Second thing could be nothing but there seems to also be another group out there acquiring lightsabers from the black market called the Acolytes of the Beyond. One small interlude about halfway into the book features them purchasing a red lightsaber from someone who claims it belonged to Vader which is very unlikely. They state their intention is to destroy it to reunite the weapon with its master in the beyond. Could be they turn into the Knights of Ren or they could be something else.