I don't think we'll play 4-3-1-2. Or not in it's traditional sense at least.
I think it will be more like a 4-3-2-1 because in Allegri's 3-5-2, there is less of an interplay with the 2 strikers. Last year Llorente was literally bringing balls down and Tevez was taking the ball off his foot. Instead, it seems there is a lone target man (Llorente) and Tevez is dropping into midfield much more than last year. So in that sense, a 4-3-2-1 keeps the attacking roles slightly more familiar.
Llorente is mostly taking up positions in the centre of the pitch. It's easier for players like Pereyra to use him as a focal point if he's in the centre. We've seen Pereyra making combinations with Llorente in the centre already.
From what I could see in pre-season it was a bit of a modified 4-3-2-1 because Tevez has more of an attacking mindset obviously. Even when Giovinco was playing the role, he was also kind of playing like an inside striker (Ronaldo style) position.
This is what Allegri's 4-?-?-? looked like to me.
---------------Llorente
-------Tevez
-----------------------Pereyra
Pogba--------Marchisio---------Vidal
and in defense:
------------Llorente
------Tevez-------Pereyra
Pogba-----Marchisio-----Vidal
So in attack, with the ball on the opposite of Tevez, obviously he can feel comfortable drifting into the middle or up beside Llorente. The only issue is when the play is on Tevez's side. Will he go wide to provide width and receive the ball looking to cut in from that side? I'm not sure if he's comfortable in that role but I noticed our corner kicks have resulted in him getting the ball back in that position quite often and he always looks to dribble to the touch line or for a shot so I have a feeling he likes that scenario.
Now if you use Morata instead of Llorente, maybe you don't want to use him like a traditional target man and you use him beside Tevez in a 2 striker setup. But I'm uncomfortable with having such little width in the formation. I think that goes against the way football is being played currently.