There's 2 systems I can envision:
System 1: A 4231 with one fullback going wide, and the other side a wide winger.
This system requires less players to bring in and uses more of the CBs (for example both Ake and Akanji are kept), really only has me bringing in Ait Nouri, Reijnders and Cherki. It's probably more realistic to accomplish.
View attachment 158173
Solid core of 21, with: O Reilly, Lewis and Echeverri as reserves to make it 24. Bernie and Gundogan out or reserves as well (but then squad is at 26).
left footed players wide left, right footed players wide right. Tricky dribblers, crossers, to get the ball into the centre for the the players to do the damage. Savio can't score, and Doku isn't finding his man on the left, swap them and turn them purely into creators in better situations.
I think this actually works pretty well, and uses the current squad to the most.
System 2: But the formation I like more is with double wide fullbacks, which requires slightly more investment (basically just a RB) and more to clear out (which is another reason its trickier), and would look like this:
View attachment 158174
Solid core of 21 again, with: Akanji, Lewis, Echeverri, Reis as reserves to make it 25. Bernie and Gundogan out or reserves (and even worse, squad at 27!)
This ones harder to do since I've added a proper RB, so it's 1 more player making the squad a bit bloated.
This doubles down on wide fullbacks and inside forwards, and hoping that Doku and Savio can also adapt to this new role (they would be backups anyways, so they have time).
I personally like this more as it's more attacking and against low blocks, I think those 3 in narrow positions behind Haaland is better than only 2 behind Haaland in System 1.