The Cubs will have 5-6 position players projected to 3+ wins
Happ
Hoerner
Swanson
Seiya
PCA
Paredes
Busch will be "fine" at 1B with 2-3 fWAR.
That just leaves DH & C. The Cubs have gotten 1.9 fWAR from their DH spot this year, which is actually top 10. On the other hand, we've gotten -0.1 fWAR from catcher, which is 29th in baseball this year. If Bellinger opts out, the Cubs almost have to add another bat on top of an improved catcher. Adding one more bat for depth would also be a benefit. The top teams in baseball have been around 30 WAR on the position side. The Cubs probably already project to 23-24 wins with Bellinger and 22-23 wins without him.
On the pitching side, There's Shota and Steele at the top of the rotation. Both are very high quality, playoff-rotation worthy starters. Steele has definite durability questions, though. Shota has also been protected throughout the year with extra days of rest, as well. Taillon has been a good back of the rotation starter or fine mid-rotation starter. Assad has been lucky to have the ERA he's had, but he's done it consistently across years now. But he does not eat innings at all and few of us would be comfortable starting him in a playoff game.
Then there's Wicks, Brown and Wesneski. They've each looked very good at points (especially Brown), but all have health and effectiveness questions going into 2025. To me, this is a rotation that is okay at the top two (with durability questions), fine at the bottom with Taillon & Assad, plus youngsters. But it has a gaping hole either at the top to push everyone down a notch, or 1-2 guys in the middle that can reliably eat innings and project as playoff starters.
For me, the big difference is the depth in Iowa. On the position side, we've got Shaw, Caissie, Alcantara, Moises, and Triantos (and Davis if he could ever stay healthy). Then there's Cam and Jonathon Long at AA. On the pitching side, we've got a bunch of injured guys between Wicks, Brown & Wesneski, plus the injured Horton.
This doesn't even touch the bullpen, where the Cubs have to make an investment to avoid the first half struggles of the past few years.
Given all that, I'd make my first investment into pitching this offseason.