It has to be on defense right? Our offence is STACKED. Even the WR room has a TON of talent, even though we haven't seen it all come together yet.
Jaquan Bailey or Jason Berryman (RIP) was my first thought. An upgrade over Peterson opposite Will McDonald. This would (and did) require opposing offenses to check those 2 guys every play or risk a sack.
Leonard Johnson also comes to mind (opposite of Anthony Johnson). 2 physical lock down corners would make opponents have to be creative in the passing game.
If I had to go offense, it would probably Lazard. He was more polished in college and would immediately give X and Milton a lot more opportunity.
The fun part of this is that it's incredibly difficult to choose because there are no obviously weak position groups. I'm holding out hope that special teams has already been addressed. We have a lot of guys competing for those very few weak spots on last year's team.