Depends on which version of Fergie (pre-2000s total attacking risk taking gambler, mid 2000s balance tactical all rounder, circa 2010s defensive tactical specialist). The different versions happened honestly because of (a) Fergie's motivations (to continue managing or retire,
notice how our teams usually drop in performances in the early 2000s, then football getting worse *entertainment wise* 2010s) and (b) his quick impressive adaptations to the generational changes in football.
I'll take the 2nd version (mid 2000s).
1. Improve coaching/trainings.
Firstly he'll bring in a good coach, 2nd assistant to be in charge of the trainings and the whole tactical side of it with his tactical input too of course.
Seriously though we're so rubbish in this aspects, and Ole made no improvements at all to it.. other than the GK coaches due to consequences which we're finally seeing improvements but that may have to do more with player management specifically gk competitions. Silly loyalty. Fergie is also loyal to his staffs, but not the crucial 2nd assistant manager whom he kept on refreshing with newer ones.
2. He'll likely bring in a real #9 first.
This for him is his most important position and normally the first one he
"fill-in". All out bid for Kane I assume or convince Haaland to join in? If he couldn't, then he'll likely play Martial there. Yes, and constantly rotate with Greenwood, Cavani etc every game until Martial hit the purple patch again, otherwise constant rotation.
3. Managing the players & Tactics
James => super sub and energy bunny, mini Kanchelskis! won't have problems playing RW.
McT => McTominator every game.
Fred => Fred the Red!
De Gea => back to God mode!
Mag + Lind => 2nd version of Bruce and Pallister
Dean => will play him in CL group stage vs the shit teams with no fear.
Romero => will let him go (sold or loan) easily considering his professionalism and good performances as 2nd keeper over the years, none of this shitty prison-ing.
Greenwood => monster impact subs across all three positions CF, RW and LW
Mata => playing as 2nd choice CAM
Rashford => right wing god
a. Default tactics? 4231
#9
Martial/James -- Bruno -- Rashford
VDB/Fred -- McT(c)
Telles -- Maguire -- Lindelof -- AWB
De Gea
Subs: Greenwood, Mata
Squad: Pogba, Matic, Shaw, Williams, TFM, Axel, Bailly, Rojo, Dean
b. Total attacking tactics? when we're going all out.
1. 433 or 4123, with one holding most likely be VDB/Fred or captain.
2. Replacing AWB with maybe James, TFM, Axel or Bailly (yes their attacking abilities or TFM's speed are really good)
3. 4114 at the last dying minutes.
c. Defensive tactics eg. in Europe or vs top teams?
1. Possibly putting Fred/VDB on the left wing.
2. Highly doubt he'll play 5 defenders system, so it'll be 4231.
3. Shaw instead of Telles is possible.
Rashford, Bruno and McTom will always be in the starting 11, James, VDB, Mata and Fred will be regulars, TFM will play more games, as does Axel being tried out even at LB. Rashford will probably play more as Right Winger to balance our right side, with AWB already providing the defense solidity at the back.
Oddly I can see Fergie dropping Martial to a regular status and Pogba as bench-warmers. And even dropping Shaw..? He wants his FBs to attack, but may be lenient with AWB cause he's young and monster in defense, confident he can improve AWB's attacking abilities. Could also see Fergie loving Rojo, he love mad bastards. Bailly I assume will play more at RB, just my guts telling me. Contrary to popular beliefs, I think he'll get fed up with Jones injuries and kick him out of the team. We're talking about Fergie in his prime here yeah. I honestly don't think he'll be bringing in Cavani nor Ighalo, but if he does, for sure he'll turn them into good 2nd choice #9s.
4. Captain conundrum?
It's very unlikely he'll turn Mag into captain straightaway in his first season. But then who knows - at that time, the choices are very poor (Maguire, De Gea, McTominay, Mata, Matic and Rashford). But remember how at that time McT is performing like crazy, McTominator form basically, and we know how fond Fergie is with McT, so it's highly likely he'll be naming McT as the club captain officially. Long term.
5. A lot of Rotations.
Seriously, he'll do a lot so you won't see our best 11 playing every games.
Our CBs, keeper, captain and real #9 (if we got one) will almost always play every games, bar injuries and the cup games/shit teams in CL.
The rest will be rotated around always, but constantly playing - that's how he got all his squad players to be useful not shit, and get back into best forms quickly. It keeps everyone on their toes, improve squad Morale with enough games and squad competitions, etc.
Just speculation/imagination IMO, based from the little information I know about Alex Ferguson, following this man's great journey.
So definitely I'll be getting many wrongs there.