If the Cubs sit tight and do nothing, they're still the best team in baseball, so that's a B for me. It's reactionary to say they can't beat the Phillies or Dodgers in the playoffs with their current roster, as their current roster is already performing better than them so far this season (and the Cubs already went 4-3 against the Dodgers this year, including 2 way too early to count games in Japan that went against them).
However, there are ways to measurably improve the team, so here's my thoughts:
1) If you can get Ryan or Gore without giving up Shaw or Horton, do it. Both are immediate and long term improvements, but if you have to give up Horton/Shaw to get them, it's not as big of an immediate improvement and you would still need extra pieces to complete the team. Just doing this would get them to A- level.
2) If that isn't an option, look to get 2 starter pieces, preferably of the Cease/Lugo/Kelly/Keller/Cabrera/Alcantara crowd.
3) Pick up an extra high leverage reliever. The pen was doing really well until the last 2 weeks, but having just one more reliable arm in there should bolster the whole pen. One of the Twins (Duran/Jax) or Guardians (Clase/Smith) options would be preferred, or Bednar from the Bucs.
4) Bolster the bench/LHP hitting. Suarez is the big score here, though I may overrate him due to how much he crushes the Cubs in particular and worry he could turn into a pumpkin at any time. More low-budget options would be someone like Yoan Moncada, Ramon Urias or Ke'Bryan Hayes.
If 1 or 2 is accomplished, that's an A trading deadline. If they do everything, that's an A+.