Thinking on it, having a system like in Stars! where detection lessens the further away you are would work only with static detectors (like cities, outposts and roads) because then players would feel like having to move individual units everywhere to maximise detection, and that would be dull and a chore, even with AI assistance in setting up patrols.
I still think this would be a good idea, especially in multiplayer a good fog or war and stealth system would be very fun. It doesn't need to be overbalanced or game ruining. Perhaps it could even be a strategy, invest strongly in stealth and remain hidden but generally weaker than others.
The other thing is the game still going to include the investment of essence by the main character into other characters? If so presumably this leaves the main character weaker. I'd think hiding the main character would be a good strategy in this case.
*edit* Thinking about it, you could learn a tech for "Way of the Forest" or something like that, allowing a newly trained unit to have a stealth bonus in forests, but with a larger cost. Same could be done with any terrain. Kinda like the hidden units in Age of Wonders 2.
Also I'd keep the system simple, if you can see one unit in a stack you can see them all.