I could see van Gaal using the 4-1-2-1-2 after di Maria's signed. It's actually just a slight variation of the 3-5-2, as the only notable differences are the fullbacks/wingbacks who are more advanced with three defenders behind them and obviously the extra centre back which could feasibly be Carrick dropping back into the defence or playing as the pivot in midfield. It would allow us to change the system easily in game and still continue with a formation that resembles the one our players are currently adapting too.
Something like this:
de Gea
Rafael - Smalling - Rojo - Shaw
Blind
Herrera - di Maria
Mata
Welbeck - Rooney
I'm really intrigued by the prospect of us deploying the diamond again, even though the last time was an unmitigated failure really. The personnel in that side is more adept at successfully thriving in that set-up. It allows us to utilise di Maria in what I and many deem his best positon, as an advanced central midfielder, while also accommodating Mata and Rooney and still having a semblance of balance in the side opposed to being top-heavy.
What's the worst that could happen? Worth a try.