Create a Commando
In BfME II the create a hero thing, which was really improved with RotWK, was a nice feature so you could fully customize a unit. In my opinion it would be nice to have something like this returning. In most C&C games yet Commandos were mostly never built as they only worked when the other player isn't watching, and paying 2000 for just a gamble isn't worth it.
-Having a unit representing your playstyle
-Maybe hard to balance
-several assets have to be done so the user has enough possibilities for customization