I think Rashford has demonstrated he is at his best in the LW/LF-position. That means we should need another striker, as Martial is too injury prone and is not consistent enough to cement his spot, and Greenwood is too raw to rely on yet. Also, 2 strikers is not enough for one position, and makes it more difficult to switch to a 2-striker system. Of the ones we have i think Martial, Greenwood and Rashford could play decently in a 4-4-2 striker position, whilst i think D.James would be better suited to a RM position in such a formation. Historically we've been at our best with 4 strikers competing in the event of 2 positions, and since Rashford and to some extent Greenwood can also play on the wings, we need another specialist striker imo. Or maybe i'm just justifying going for Haaland
Also, we need a RW competitor for D.James. The usual suspect is Sancho ofcourse, though i'd guess that is a more likely summer transfer than a January one. That likely requires us to qualify for CL i think. Dunno if we have any other british RW targets that are likely to be a significant upgrade on D.James. McNeil? Cantwell? Not really generational talent, and not really much better than James i'd say. Bale on loan is naive?
Lastly, but maybe most importantly, we need a midfield anchor. Someone with great positioning and ball winning ability, along with good passing range. A slightly stronger and faster Carrick would do. Even though he has some way to go to fill those shoes, i'd say Declan Rice fits the young&british category player that OGS has gone for so far. Doubt he'd be available until summer, and would require a relatively hefty price, much like Sancho. I doubt OGS consider Garner ready to "replace" and improve on Matic and/or Fred, considering he hasn't played more.
Now, those players, while expensive, i think are gettable. Lets say a total fee of £180m-220m, provided the release clause for Haaland is to be believed. I think that is doable. We'd likely look at some outgoing transfers aswell, like Smalling, and maybe Pogba. Maybe even Bailly, Jones or Rojo?