I have already expressed a similar idea in my poll about heroes. Then 31% of respondents chose the option of a separate mode with heroes.
Your concept better reveals this particular idea. And I totally agree with it.
The only thing I would like to add is not limited to narrative differences. I think a separate game mode with heroes could be it is focused on Xbox players. Devs can make the gameplay around heroes, reduce the limit of units, change the control. Instead of adapting the main mode to the gamepad, it would be better to create an adapted mode for gamepads. And devs could use heroes in it.