#1. Nico Hoerner
Although he's part of the heart and soul of the Cubs, Nico Hoerner is maybe the best candidate among the established core to be shipped off at the trade deadline. Unlike Bellinger or Happ, he doesn't have the same contract concerns as he's earning $11.5 million per year through 2025 and $12 million in 2026, all without a no-trade clause.
Hoerner has been around league average on the season with a .248/.334/.348/99 wRC+ slash line, though he's picked it up more of late with a .265/.346.408 line over the last 14 days and 55 plate appearances. On top of that, he's continued to provide baserunning value with 14 steals. A track record of average to slightly above average offense with a low strikeout rate combined with the ability to play second and shortstop at a Gold Glove level should make him a valuable target for teams looking for a stable middle infielder for the next few years. Even in a year where he's made more fielding errors at second base than ever before in his career, he's still sporting a positive 5 outs above average on the season.
Losing that defense up the middle would be tough for the Cubs, but it would also open up possibilities for the future. James Triantos, Matt Shaw, Jefferson Rojas, and more are rising up the farm system as potential middle infielders. If they wanted to get creative, Michael Busch has experience at second base in the minors and could slide over, clearing the way for, say, a Pete Alonso signing if the team wants to stay aggressive.
Of course, moving Hoerner shouldn't be done for the sake of selling. His contract is affordable and he could still be valuable for the Cubs if the hope is for a quick bounceback. If the right deal comes along though, he'd be one of the easiest to trade for significant value, even if it would sting to do so.