I don't think Allegri will play 4-3-3 using regista, this will be used probably if there is no one to play as AMC (Dybala, Fagioli and Kulu aren't available)
I think Allegri will play 4-2-3-1 with Fagioli as Dybala's backup, if he want to use Dybala then this is the best system to adopt, Kulu can play AMC too but he will be getting more time on the right winger. And Fagioli will have more time too because he will be Dybala's backup
the difference is Dybala play as second striker that go deep, but Fagioli playing as central midfielder that go up the pitch. But all in all that will be 4-2-3-1
But whatever the system, Locatelli can play as regista too, he can play as mezzala on both side. So he can play on the left with Arthur on the right
or he can play on the right with Rabiot on the left. And if he really come to us, then he will be our 1st choice as central midfielder