A diamond, no? This would work there, but I don't think we would be using that every match. In a 4-3-3, Pogba generally wins out, and if we get a B2B (Lemina links) and someone behind them (Longstaff/Matic), I don't know where that would leave Fernandes should we get him.
Fernandes is an excellent option should Pogba leave, otherwise, I don't think he will be too happy with being second fiddle (nor should he, it's bad for his career). A midfield of Fernandes and Pogba would work had we gotten someone like Ndombele, who covers a lot of ground while also moving forward (a junior Kante, if you will). With either Matic or Longstaff, they're more static playmakers from deep, so that wouldn't work properly, at all.