Looking back, i agree with your point about us lacking one more player to control the game. Bernardo Silva would have been great to have on the right, as he moves inside and links midfield and attack.
Martial, Rashford and Lukaku all certainly benefits from a 2 striker system.
I still think 3-4-1-2 is too open in midfield, but if we could get a 8/10 hybrid who is comfortable defending, so that in defensive phases he moves back in CM making it a 5-3-2, that would be ideal.
The thing is, playmaking number #10's are a bit dead. Players like Eriksen, David Silva and Ozil are rare. I'm not sure you will find the player you are looking for, unless you go for the mentioned, but you're not a fan of Ozil, and Eriksen would be very difficult to get out of Levy's grip.
We have shown that we can create plenty of chances with a front three of Martial, Lingard and Lukaku, as long as Pogba is fit, so a player like Savic could be a good option. Pogba and SMS would then switch between helping in the build-up and bulldozing forward, as they share many of the same qualities.
Martial Lukaku
Savic Pogba
Sandro----Matic----NewRB
Rojo Vic Eric
DDG
With the 4-3-3, players like Parejo or Seri would probably be better fits than Goretzka and Saul. But besides them, it's difficult finding CM's with playmaking capabilites. And it would require a change in style of play, bit more possession orientated, which Mou won't like.