I know this is a bit controversial, but I think Matic and Fred with Lingard ahead could be our best combination in the middle, purely because I don't think we'll see the best from Pogba when he is not the one given the most license to go forward in a midfield 3.
I say why dont we play a narrow 4-4-2 formation.
For example if we can sign Sandro then I'd like to see us line up like this next season:
De Gea
Valencia-Bailly-Smalling-Sandro
Matic
Fred-Pogba
Lingard
Lukaku-Martial
Realistically though Jose would play Sanchez over Martial and maybe Jones over Smalling (or Bailly?!?!?)
Yeah, I like the idea of a diamond as well, however I am not sure Jose fancies having two strikers upfront. A slight twist to that, which I would really like to see would be a 4-3-2-1, almost a pyramid like Shape, where you have 5 midfielders packing the centre. But like you said, you would need a genuine wide player at full back who is capable of crossing, beating his man and obviously, very comfortable with his defensive duties.
Team would look something like this
DDG
Dalot-Bailly-Smalling-New LB
Fred-Matic-Pogba
Lingard-Sanchez
Lukaku
It's not vastly different from Jose's 4-3-3, but we don't really use a front 3 with typical wingers or inside forwards.
I think this formation would bring out the best in Sanchez as well, since he will be more central and much like Lingard, can cause problems with his movement and incisiveness. The two Attacking Mids will have the freedom to roam the entirety of the attacking side of the pitch being ably supported by a more rigid trio. It will give Pogba the room to move front and occupy spaces vacated by Sanchez as well and the same applies to Fred. Especially considering the likes of Sanchez and Lingard are not averse to tracking back, there is a certain safety net for someone like Pogba in case of counter-attacks.
The obvious problem is defending in the flanks, where the full-back is likely to be outnumbered. But I think if we have a compact back four with the inner CB covering a full back and obviously Matic going very central and playing as a 3rd CB almost, combined with a high defensive line and if De Gea finally does change his game to be more aggressive when there are loose balls near his box and play a sweeper-ish keeper, we could still be very potent defensively.
Just brainstorming though. Sanchez and Lingard and Rashford too always try and pick out Lukaku, so having them in areas closer to the striker would help in the continuity of the play rather than trying ambitious through balls or crosses which are always a hit and miss with those three.