My guess is that we'll see the return of 4-2-3-1.
Excellent call.
From what I saw in the friendly yesterday, it is the 4231 off the ball that shifts into a single-pivot offensively. Normal back four with Dest at LB, a Nico-Frenkie double with Gavi in the 8/10 hybrid role.
All things considered, I think Nico absolutely needs to be in the squad as the ?physical MF? signing, as he will already lighten the load on Busi and complements Frenkie nicely, despite the two being similar.
The difference between this and how he did it last year is that the attacking 3 only had one player (Gavi) looking to sit in the hole while Depay and Demir were both willing to go wide, make runs in behind and stretch their lines. Unlike a Grizzy-Messi-Coutinho 3 where all three want to sit and create in the hole. Its better tbf, we will see how it translates when the season starts but I see why he wanted Depay so badly now. He is immediately giving us something we have lacked for a while: vertical threat, have not had it since Ansu went down.
Where it gets complicated is if/when Messi returns and where you play him in this system. In my humble opinion, it really will not work in the tough matches and I question the merit of forming a system that is not sustainable for your best player. It does not make sense to play him at RW/RAM because of how central he likes to play and then the RB gets outnumbered on the right. If you put him up top again, why did you sign Aguero then? He puts a lot of pressure on the 8/10 to be a legit goal threat when Messi shows to the ball in this role and really the only player suited to this is Grizzy and we saw last season that this duo up top is mediocre but not good enough. Lastly in the 8/10, Koeman needs this player to be able to support the pivots off the ball in defensive situations and this drags Messi away from goal too far.
If he is going to play a 4 back system, I basically think we need to play an offensive 433 with two true 8s a la Pep with him at RW/false 9 or keep a double pivot and play with a 3 back system to allow Messi the cover defensively to have a true free role. Trying to blend the two with a 4231 is just too easy to counter tactically, from either crafty La Liga teams or the European elite.