Here's my submission for a new and improved Mobile Defense doctrine.
1) Osttruppen which are available at 0CP and can quickly fill the frontline in a defensive role, in accordance to the theme.
2) Mortar Half trucks, providing indirect fire wherever the opponent pushes, more mobile than stationary mortar teams, in accordance to the theme.
3) Tactical Movement, allowing fast response and repositioning to where the opponent is pushing, in accordance to the theme.
4) Assault Grenadier Reserves, allowing rapid requisition of two shock squads to fill sudden gaps in the army roster and sprint/tac movement to wherever they are needed, in accordance to the theme.
5) Panzer IV command tank, mobile and able to assist in defensive operations, in accordance to the theme.