My assessment from an outside point of view:
Players I think are definitely staying and definitely playing regularly for you next season:
Maguire, Wan-Bissaka, Shaw, McTominay, Martial, Rashford.
Players who might leave but will definitely play regularly if they stay:
De Gea, Pogba
These will need replacements if allowed to leave.
Players I think are staying but might only be squad players:
Lindelof, Fred, Lingard, James.
Players I think are staying but will definitely only be squad players:
Dalot, Tuanzebe, Fosu-Mensah, Greenwood.
Players who might leave but would probably be squad players if they stay:
Grant, Henderson, Romero, Bailly, Smalling, Jones, Young, Matic, Mata, A.Pereira, Sanchez.
Players who will likely leave and would definitely only be backups if they stay:
J.Pereira, Rojo, Darmian.
Youth players just about on the fringes of the first team:
Garner, Gomes, Chong.
(Out of contract players: De Gea, Grant, Bailly, Young, Darmian, Fosu-Mensah. Also Matic but has +1 year option)
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Firstly, De Gea and/or Pogba will need quality replacements if they leave;
I don't think Ole & co will choose to rely on Romero or Henderson for the #1 GK spot if De Gea leaves, more likely one of them will leave (sold or loaned) and the other will stay to be #2 behind a new GK. Try for Oblak or Szczesny if attainable, otherwise someone like Strakosha or Onana. PL experience option - Schmeichel jr. I think that's the minimum level you would want to go after with the intention of challenging for top trophies in the next 2-3 years.
If Pogba leaves you need someone who can play a box-to-box role at a high level, someone who can contribute in all aspects of the game in the middle of the pitch and also create and score goals. If it was possible to lure Eriksen I think he'd be to optimal choice, if not ask RM about Isco. Otherwise I think Bruno Fernandes (linked with you this summer) or Donny van de Beek would also be great.
Secondly, fill out the weakest links in your strongest XI (excluding potential De Gea/Pogba replacements) ;
From my point of view those would be one forward (mainly right sided), one midfielder (holding mid), a left-back and a centre-back, and I would prioritize them in that order.
For the right sided forward role I think Sancho or Ziyech would be great options, with the latter being my favourite (more versatile, more attainable and more affordable). Other options could be Neres and possibly Felipe Anderson for someone with PL experience.
For the holding mid role I'd probably go for Ndidi. He's quite good, got PL experience, and he's so young he still have lots of room for improvement and could end up staying for many years. Other options if attainable could be Weigl or Thomas Partey. A riskier option could be Sander Berge from Genk who I think is a big talent, although he hasn't got big league experience.
At left-back you could go for a cheap-ish young talent as backup/rotation for Shaw or a starter depending on where money is prioritized. If you go for a starter I'm not sure there's a whole lot of options on the younger side. There's Alex Telles and Tagliafico who both turn 27 this winter and there's Chilwell. If you wanna go for a younger talent type player I'd have a look at Vinagre at Wolves or Jamal Lewis at Norwich.
At centre-back I think you probably need to offload a couple of players before signing any new ones unless it's a cheaper option (young talent or experienced out-of-contract player). I think Bailly's contract is up next summer(?) and you need to get rid of at least one out of Rojo/Smalling/Jones as well, preferably two, before spending more money on a new CB. There are some top class older CBs with contracts running out next summer; both Vertonghen and Alderweireld at Tottenham, and Thiago Silva and Chiellini. The latter two will both be close to 36 years of age though, and haven't played in a league with the intensity and pace of the PL. Huge money signings at CB like Koulibaly or Skriniar I don't think will happen with all the money you've spent on CBs the last three years (Bailly/Lindelof/Maguire + new big contracts for Rojo/Smalling/Jones). If he's affordable I would try and get Nathan Ake. Otherwise I'd go for someone like Ruben Dias or Mario Hermoso.
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Assuming you get De Gea to sign a contract extension and convince Pogba to stay I think something like this could make for a good (and achievable) summer;
In: Alderweireld, Chilwell, Ndidi, Ziyech. (+new contract for Fosu-Mensah and possibly Grant)
Out: J.Pereira, Romero, Bailly, Rojo, Smalling, Young, Darmian, Matic, Sanchez.
New team: ............................................. | Rotation/backups
------------------- De Gea ---------------------- | Henderson, Grant
- AWB - Alderweireld - Maguire - Chilwell - | Lindelof, Jones, Shaw, Dalot, Fosu-Mensah
-------------------- Ndidi ----------------------- | (McT), Garner
--------- McTominay ---- Pogba ---------------- | Fred, A.Pereira, Mata, Gomes
------ Ziyech --- Rashford --- Martial --------- | James, Lingard, Mata, Greenwood, Chong
I think with that squad you'd be only one more summer window (and possibly necessarily one better head coach) away from challenging for trophies.