We cannot play with 1 striker. When we do so we won't be pushing the opponents players back enough. I honestly believe a modified 4-3-1-2 in a 4-4-2 diamond midfield maybe our best option.
Melo and Sissoko are not good passers to be paired, Sissoko can play next to Tiago, Marchisio this way he can do the dirty work while his partner (Tiago or Marchisio) can concentrate in delivering the passes.
We lost alot when Nedved retired, he was capable of playing both as AM and LM. Thats what made him so successful during the Lippi era because he used to start at the left and go the center or vice versa. Additionally, he had defensive capabilities.
Dropping Del Piero because we want to play 4-2-3-1 one is not sane. We need him so much on the field, thats why we need to play with two strikers up front. And yes, there is nothing wrong to adapt your formation depending on your players. We have the players to do a 4-3-1-2 or a diamond 4-4-2 lets stick to that. 4-2-3-1 makes us look so immature. With Marchisio back in action there is absolutely no reason not to play 4-3-1-2
During Defense:
Buffon
Cacesras Cannavaro Chiellini Grosso
Camoranesi Sissoko Marchisio
Diego
Amauri Del Piero
And during attack
Buffon
Cacesras Cannavaro Chiellini Grosso
Sissoko
Camoranesi Diego Marchisio
Amauri Del Piero
With the full backs pushing forward.
This is very close to how the Diamond 4-4-2 works.
Giovinco and Del Piero will rotate on the Assistant Striker position, while Amauri/Trezeguet/Iaquinta on the striker and if needed they may cover for assistant striker.
Giovinco and Del Piero can also cover for Diego if needed.
The LM/RM can be covered by Camoranesi, Tiago, Marchisio, and De Cegli. Camoranesi and Tiago are the weakest defensively thats why I would not pair them together unless it was definitely necessary.
The DM: Melo, Sissoko and Poulsen. The three are not great passers, although recently Poulsen has shown some hope. Thats why it would be a midfield suicide to pair any of the three.
LB: Grosso/Molinaro
RB: Caceras/Grygera/Zebina
CB: Cannavaro/Chiellini/Legro