Well, we are planning on implementing reinforcements that can be sent in from the moment you're suffering losses with your starting army.
This would really lenghten the action seen in an average game and contribute to the experience.
And in theory it's very simular to your idea, you won't have all your units ingame right away, you'll decide the forces you start with (worth 30 pop cap.) and then when you're losing units and when you believe you can still have reinforcements make a difference, you can call pre-defined units in to aid out.
(But ofcourse, if you're sure you stand no chance, even with the possible reinforcements, it's better if you don't send in those reinforcements just to see them getting killed
)
But anyway, thanks for contributing with this idea!