There is one already for aoe2 as the video below shows, and I imagine an enthusiastic modder could easily make one for aoe3de since the map and units are already in 3D 
It would be so dope to charge into an army as a hussar or carefully shooting between the trees as a skirmisher (Yes, like many other shooters out there, just in the aoe3DE world and logic)
So, anyone seen any mod like that? Or if I turn out unlucky, any game suggestions with the same thematic as aoe3?
Giving it more thought, it could combine both 1st person combat as well as the rts commands of the game, just like the explorer could be the “Leader” of the settlement (something of a combination of "totally accurate simulation).
Even cooler could be checking a fort from the inside 
OOOOOOOR, even just a simple “Possess” mod could be cool, watching the battlefield live from the view of a foot soldier.