There are a lot of defensive players that would improve the Colts right away. You have cornerbacks like Quinyon Mitchell, Terrion Arnold, and Cooper DeJean. You also have edge-rushers like Laiatu Latu or Jared Verse, that would provide some much needed pass-rush from the edge. Any of those five players would be competing for a starting spot right away and would also elevate the ceiling the defense can reach. Keep in mind this is a unit that was among the worst in the NFL last season at preventing points (28th in scoring defense), and slightly below average when rushing the passer (23rd in pressure rate).