Surprised to see a lot of Pogba answers, Pogba can hardly dictate the play, he can't even dictate the play for 90 mins, then regularly week in week out..
He's the best passer, there is no doubt but he could impose on the game much more, keep it simple so we could get a bit more possession, we lack control of the game hence, it's hardly to think about a player makes us tick, doesn't look like there is a gameplan either but goes to manager, with a stronger, forgive me the term, philosophy, he could do better for sure, but his position is a bit advanced too to makes the team work, he's good in flashes with an extreme highest level, which is basically our biggest problem, we have too many players being good in flashes. That's why I think that players like Mata, Herrera are important.
In fact combo of Herrera and Pogba could possibly match the criteria, Herrera keeps it simple, very good at one touch passing, does a lot of running, wins the ball and have a ocassional nice through ball, that allows Pogba to do what Pogba does - creative passing, taking on players in midfield, running with the ball. Pogba alone doens't make us tick, right now he's too tenacious and would be a bit of waste to play him deeper to make the play there but with a strong dynamic partner, it could be a great combo, I am not talking about Matic obviously who for me, even though he did well this season overall, won't get the best out of Pogba as he's too similar - trying to hold on the ball for too long and look for the pass, too static, even though being more defensive..
So it will be interesting who we bring into the team but I don't think we can match the best teams unless we make our team tick like you said in OP and that requires dominating the midfield, not losing it to any a bit more pressing team, and that requires 2 great dynamic midfielders who can pass and defend. That means a bit more Herrera at the expense of Matic, or at least midfield three more often than not.. and a bit more fluid and aggressive system, which will always be a problem under Mourinho..