Here's another idea.
Anyone play Stars! (how to put a question mark at the end of that question without it being absurd?
)
One really cool thing with Stars! was that the detection was based on strength or the radar, distance from radar and base unit stealth. How about a similar system? Forts and big towns are capable of differing levels of detection. However the further from the town the less strength the detection has. This is where stealth comes in. Units with no stealth always get spotted. Units with stealth get spotted once the level or detection becomes greater than the stealth, and of course the level of detection becomes greater the closer to town or fort the stealth unit is.
I think this works well because it allows the "stealth raider" tactics of Dom3 to play out, also it fits in well with the smaller army size feel CoE2 has over Dom3.
There has to be a cost to increasing detection. A direct investment like an "upgrade" button would be the simple way, the question then is if you lost the town would the enemy retain the benefit? Or I thought a system where you move a slider to reduce town income to improve detection, but how would this work with forts which have no income?
Including this would work with "flavor" too. Some factions have better (or cheaper) detection than others, and some factions have better stealth than others. Magic could increase or decrease either.