First of all our midfield is stacked, and we'll NEVER use a 2 man CM partnership to make room for teenage wingers or half decent players who are being shoe horned on to the wing, that's just $#@!ing not going to happen. It would take a retarded coach to take our best area and weaken it so we can play Coman and Pereyra AND drop one of Tevez/Llorente. Not happening.
Second of all, Allegri has never ever used anything remotely close to a 3-4-2-1, hell only Napoli from recent memory pulled it off with any success, because their attack was stacked and their CMs were average at best.
So I don't want to read more $#@! about some crap 3-4-2-1 when we have 1,000 midfielders and our best winger is a teenager.
As for 4-2-3-1, come back when we have a top class AM and 2 top class wingers. Otherwise we'll be fielding some disgusting monstrosity with half a team full of players who are half decent in a 4-2-3-1 system, we'd be lucky to finish top 3 with our current team in a 4-2-3-1