I think the Cubs can realistically greatly improve themselves without touching their top 4 to 5 guys, depending on where you have Alcantara.
Merrill Kelly- reliable mid rotation guy, he doesn't have the big upside but he's safe and you feel just fine with him starting a playoff game. You can probably get him for maybe a Gallagher or Triantos type, I can't envision him costing much more than that.
Sandy Alcantara- This is all depending on the ask. If they're willing to take a headliner of Alcantara go for it. He provides the upside Kelly doesn't so even if he falls on his face it's fine because you can get him in the lab this offseason and you still have 4 other starters who can make playoff starts not including Horton.
Willi Castro- He's just such an obvious move that I'm really gonna be blown away if he doesn't end up on the Cubs. He checks all the boxes. He has lots of positional versatility and he mashes lefties. I can't imagine he gets someone in the top 10, probably more 1 in the 11-20 range and another from further out that Minnesota is high on.
For reliever, I think the Cubs messed up not getting Robertson for just cash, but I'd go for Shelby Miller. He might cost you a top 10 guy, but I think it would be back end of the top 10 so maybe a Birdsell type.