He's not going to tell you what the idea is, of course — he is J.J. Abrams, after all. But if there's a person in such a position to actually get a movie made about the mysterious cronies of Kylo Ren made, it's him. We still know practically nothing about the Knights of Ren, the strange order the young Ben Solo pledged himself to become Kylo Ren — we barely even know what they look like, given we saw them for about a second in The Force Awakens, and they were shrouded in stormy darkness at the time. We have a few theories about how they came about after Return of the Jedi, but it's definitely a huge area of potential for Star Wars media to explore.
J.J. Abrams agrees, apparently — at least, when asked by Collider during press for Star Trek Beyond about what sort of spin-off movies he'd like to see joining Rogue One and the untitled Han Solo movie, he says he has a few thoughts about a Knights of Ren movie:
I gotta say Kathy Kennedy, who's running it all, she seems to be pushing all the right buttons. There isn't something that I'd love to see — I guess the one thing I would say is there's a Knights of Ren story that I think would be pretty cool to tell.
Usually these sorts of news stories — about the shocking fact that people in Hollywood have ideas for things and would love to be gainfully employed to make them a reality — are rarely exciting. But chances are that if the director of The Force Awakens has an idea for a stand-alone Star Wars movie, Lucasfilm president Kathleen Kennedy would be very interested to hear it.