There's a lot of interesting ideas going around here. Personally, we should forget past sides, especially under Fergie, as the game has changed substantially. I think it also depends on the type of football the team wants to play. If we looked back to Carrick, and maybe even Matic currently, both became dedicated DMs known for their ability to read the game clearly and provide a line to intercept rather than put in strong tackles. However, Matic was capable of doing both, Carrick was never one to be that hard tackling DM. These players are the likes of Busquets, Schweinsteiger and Alonso. Currently, Matic does the role for us, but his forward passing has become increasingly suspect. The likes of Fabinho and Fernandinho are the best of both worlds, naturally playing for the two best clubs at the moment. Both can play that destroyer role for that club, as well as being able to act in a role of playing forward in front of the defence.
Casemiro, who is probably the best DM in world football at the moment, Ndidi and Rice are definitely destroyers. Casemiro does have a decent pass, but he's never gonna split the play in a forward role.
Every team who wants to play at the top level has to use a 3-man midfield, else run the risk of being overrun. I think Pep is the only one who recently experimented with Inverted fullbacks, who moved into midfield instead, back when they had Delph in the side.
I've seen people suggest Saul Niguez, but he's always seemed better in an offensive role. The thing about Atletico is they also sit deeper than we would, so there's more cover, and he usually has Partey alongside to help out.
Neves is the other one, and he's easily one of the best passers, but he's not mobile enough, and is definitely suited with a more mobile hard-tackling partner.
In terms of our squad, I think we need a younger, more mobile Matic. Basically, someone who can put in a strong tackles, provide a shield by himself(Matic struggles these days due to the mobility issue), and can still play the ball forward as Matic did in his younger days. Don't know why that's gone awry at United.
Reason is we have Bruno in the most advanced role, and if Pogba stays then you have one hella creative midfield. But then you need to protect them. Think Madrid with Kroos and Modric, with Casemiro to protect. Chances are the emergence of Valverde once Modric retires means they'll move away from Casemiro in favour of a more creative midfielder.
I don't understand why we'd sign Grealish to replace Pogba, because they'd occupy the same position. Someone in a creative advanced, but not goalscoring role would be more adequate. Someone similar to the way Tielemans sits and controls for Leicester. Perhaps Neves or Fabian Ruiz? We need a Scholes or a Xavi, basically, to dictate the game. If Pogba stays, he is more than capable if he would just damn well remain disciplined.
But, back to DM, I like Ndidi, although not the best passer, but he has great room for improvement. Allan may be another option, but when I watch him for Napoli, he loses the ball a lot. Vidal is also available at the end of the season, but I'm not so sure. I don't know too much about Zakaria either, but he seems to have great opinions of him floating about. I can't see Saul playing that role, so I'd have to go with opinion and think Zakaria would be the best option.