The team is quite good to be fair. I'd say the main things to sort out are
A -
A clear structure and automatism in the players runs. Louis van Gaal is great at this but when it's all in order his clubs tend to need a better man manager
B -
A collective cause and ambition as well as leadership. Ole seems to have the players behind him but he could do with another Bruno, i.e. a Zlatan, Haaland, Ribéry, Ramos-type that won't accept failure. Look at Kimmich who I absolutely love as a player. Without his resolve he'd be half the player, Bruno is the same, Schweinsteiger before them as well. Liverpool have several such as Milner, Henderson, Lovren (before).
C -
A 40-goal a year striker and two 20 goals a year wingers. No matter what team or league you can't have Bruno (CAM, 26 goals) and Rashford (LW/RW, 20 goals) be the top scorers. 30 is the bare minimum and I'd say you need 40 goals a year from a top striker today across all competitions in order to challenge for the league and the Champions League.
Zlatan Ibrahimovic (28 goals, 16/17) and Romelu Lukaku (27 goals, 17/18) came close, Zlatan would actually have made the mark but for injury. The last time a United striker struck 30 times was in 12/13 when van Persie draged United to the title. The year before Rooney scored 34 and United lost the title on goal difference, tough luck. United already have Rashford, the other two slots need to be sorted out. Haaland is the obvious one but perhaps out of range.
I'd take a look at Haalands replacement at Salzburg -
Patson Daka. The 22-year old zambian has scored 31 in 37 this year, 27 in 45 the year before.
Andre Silva at Frankfurt also fits the bill as a young, talented striker.
Andrea Belotti at Torino could be your 20-25 goals a season striker as well as providing a good foil for the wingers, perhaps not quite the fire sparks United need though. I'd also look at
Jamie Vardy. He is criminally underrated and could well be on par with the likes of Eto'o, David Villa or Pippo Inzaghi if provided the opportunity. Perhaps he has another two good seasons in him?
D -
A deep-laying playmaker. You need one midfielder who can run, score the odd goal, harass the opponents and preferably be rather tall as to be of help offensively and defensively. Seydou Keita was great, Leon Goretzka does incredibly well, Casemiro is legendary, Thiago Motta was another one. McTominay fits the bill. But who should his partner be? Where is Uniteds Kroos, Thiago, Verrati, Pirlo, Scholes?
I'd look at someone that's approximately 26-29 that still has the legs but above all leadership and experience.
It hasn't worked out for Thiago at Liverpool, try to get him if possible. It would be quite the coup. Unfortunately I don't follow football as I used to but i remember
Julian Weigl of Dortmund being touted as the next Busquets when he broke through at Dortmund. He is only 25 and plays for Benfica, if he hasn't regressed or stagnated he might fit the bill. That type of intelligent player often rise to the occasion and play better with better players around them.
Max Arnold was also hyped up around the same time but never got his big time move, according to Kicker he has had a great season at Wolfsburg. Today all the rave is about
Florian Neuhaus, 24, at M'Gladbach. If Bayern can afford it post-Covid they will buy him but if they can't he might move abroad seeing as M'Gladbach always tend to nurture players and then sell them on.
Kristoffer Olsson who plays for Krasnodar in Russia and the Swedish NT is a Modric-replica. Great buy!
E -
Get rid of Maguire. The man wears his heart on his sleeve but he is simply not good enough, Smalling is better. In the years that I have followed football I've always had a sweet spot for the great players emerging from the Sevillas, Parmas, Southampton or M'Gladbachs. As a general rule of thumb I believe german newsoutlet Kicker are right 90 % of the time when they rank the players in the league. The players are ranked as World Class -> International Class -> National Class but it often happens that no players in the league are deemed as World Class. It lends credibility. Such is the case this year when last years recipient of World Class (David Alaba) fell out of the list all together after a lack of form due to contractual issues. I haven't watched this guy play but the no1 pick in the league, above Mats Hummels (2), German NT-starter Mathias Ginter (3) and Bayern-bound Dayot Upamecano (6) is
Burkina Faso and Bayer Leverkusens Edmond Tapsoba. He is 1,94 (6,5) and looks solid and technical on Youtube, mind you I haven't watched him play but I really do rate Kickers analysis. He just turned 22 as well.