There aren’t a lot of free agents from MLB Trade Rumors’ Top 50 list who remain unsigned (although, of course, Cody Bellinger is one of them), and one area the Cubs definitely need to improve is the bullpen. It can be reasonably argued that with a better bullpen, the Cubs might have won as many as 90 games and made the postseason in 2023 (their Pythagorean projection had them as a 90-win team).
The Cubs already have a closer, Adbert Alzolay, so where they need to improve is in the setup area of the pen.