My point is that it's already a part of Ostheer mostly. Maybe we could have a doctrine like that, but Ost already has 2 Mechanized doctrines.
Mechanized in name yes. This fantasy PE and OKW mashup faction would play quite differently from Ost (Though mechanized assault has some superficial likenesses, it would play out quite differently and with a more locked tech path). Early game you would see one or two light half-tracks mixed in with the dynamic between long ranged light infantry and the sturmpios.
This would then morph in to a infantry centric game supported by light vehicles and assault guns. And unlike Ost, it would actually be possible (and probably advisable) to use panzergrenadiers as the real backbone of your infantry force as the game progresses and they will have to be supported by light vehicles or assault guns/tank destroyers. Rather than having the light vehicles or early armour function as shock units (an exception to this would be the 2cm 250 since this would basically be a slower but sturdier scout car)
And unlike both Ost doctrines with mechanized in the name the endgame here does not include a heavy tanks as linebreakers, so this one is still based on teching to reach a real tank, and then its a panther that still needs plenty of other supporting units to bring home victory.