if you want to make it as a single player game, it think it would be easier, both to design and play, if you make it with a medieval/roman/barbarian ambientation, it would be just like in movies, when you see the general fighting as another soldier and giving orders at the same time (thinking of Gladiator or braveheart).
In actual wars, the men with the control aren´t at the battlefield, because control tasks take all their atention, making this as a FPS you will only have a strange system of changing between menus, as long as you will never go into battle to take care of the whole action and you will be only moving between the different buildings or menus in you remote-control managing system.
What i say is to rip all the tasks that are not directly related to the action-point-of-view, as resource gathering, unit training,... Just imagine you are Aragorn at the top of the walls of helm deep and you have to command the troops, you just don´t have time to care about the haversts of Rohan.
Similar things have been done, but at a smaller scale, for example, in Swat3 or rainbowsix, you are another soldier, but you can give orders such as "follow me", "hold fire", etc
With your intention to make it on a larger scale i think it would fit better with large armies of knights, archers, that you command with quick orders given to squadron leaders.
Managing tasks could be also be very interesting and well integrated if played between the battles, this could make the game deeper and more playable at a time.
Well, this is just my idea, hope it give some good idea to you.
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Ionicboy
http://www.ilogicgames.com
