Leonard Williams is an interior defensive lineman, like Muhammad Wilkerson and Sheldon Richardson. Quinton Coples is too, but he's been relegated to outside linebacker. So how will the team fit Williams on a line that has Wilkerson, Richardson, and Harrison? A number of people have jokingly suggested a 4-2-5 defense, but Todd Bowles has said that the team will remain primarily a 3-4 base, even with the addition of Williams. Something's gotta give.
Obviously, the team won't be exclusively 3-4. We'll see some 4-3 plays and so forth. But between Wilkerson, Richardson, and Harrison, who is the odd man out? Assuming Wilkerson isn't about to be traded, making this article irrelevant, it seems likely that Harrison will see his snaps limited. You may be surprised to know that Harrison actually only played 47.6% of defensive snaps last year. He's an elite run defender, but he offers little as a pass rusher. He's basically just a wall that doesn't move, for both good and bad.
So the result seems to me that the team will likely keep Harrison in on first down to stuff the run, then replace him with Williams and swap the defensive around, using as much versatility as they can. All three players can play the five technique, the three technique, and even the zero tech. There isn't much any of them can't do, and you can use them interchangeably. All can stop the run and all can rush the passer. There's a lot to be excited about with the addition of Williams, and this is certainly a good problem to have.