#3 Jack Lambert, Middle Linebacker
There are a few other linebackers you could consider here: Ryan Shazier or Jack Ham. However, If you add Lambert, you're getting one of the best to ever play the game. In fairness, what you get in play-making ability, you might lose in penalties as Lambert's play style was crafted for a day when you could hit in ways that would be illegal under today's rules. Although with the rules adopted back then, he criticized those, when he was quoted as saying to put the quarterback in a dress. Then again, Mile Tomlin didn't draft Broderick Jones because he will kiss the defensive lineman.
Putting Lambert on the 2023 Steelers team would be an automatic upgrade in any way possible. You could drop him into coverage. He could cover as well as any defensive back. You could have him blitz, stuff the run. He and T.J. Watt could ruin the days of a few quarterbacks. On every snap, he played any time the defense tacked the opposition; usually, Lambert was in play in some capacity. He has an awareness few other players have. He even had textbook tackling skills. He would raise the level of play of anyone around him.
If you put Lambert on this defense, it's hard to not see it function at a much higher level than it will. He would improve the defense so much that it's implausible they at least do not make the AFC Championship game. Mike Tomlin has a particular affinity for linebackers; it appears each year he keeps burning through linebackers until he finds one he likes. Joe Schobert, Myles Jack, and now Elandon Roberts. So think how much Tomlin would love a physical player like Lambert in his defense.