Ok, so from what I gathered there, you pretty much set up what I was looking to do, nearly verbatim.
However, I'm also looking for some of the following details:
1) any unit CAN be invisible to another based on the most basic form of visibility/invisibility categories (say, stealth perhaps).
2) This stealth value would default to 0 but all units would have a stealth value regardless. Obviously, units designed to elude detection such as rogues and spies and such would start with higher base values that would increase with upgrades. But many animals would have higher base values as well.
3) There would be basic stealth promotions available to Scoundrels and Spies that would powerfully increase their stealth value. Other units could gain some access to these too... units like Skirmishers, Special Forces, Mercenaries, Some trained animals, Recon...
4) Promotions to enhance stealth when the unit is on specific terrains such as Forest etc... More widely available.
5) Promotions that enhance stealth by round fortified (Blend In ability)
Available to most units that can fortify.
6) Terrain itself would naturally either increase or decrease the stealth value of a unit. Marsh alone could possibly hide even a large army from the less vigilant.
7) Distance from observing unit should play a role: -1 stealth if only 1 spc away, 0 if 2, +1 stealth for 3, +3 for 4 etc... obviously the visibility range is also going to be an ultimate limitation.
8) Improvements could make stealth values higher or lower (higher if an improvement was a hiding hole for example, lower if its a village where people may see and report you to authorities.)
9) Then there'd be an Alertness value on all troops that would play a factor in this. I believe you basically have the setup for all of this established in the previous mod already... And of course there'd be promo lines, and equipments like spyglasses and night vision goggles, for Alertness as well.
10) City Buildings and Fort Upgrades should also increase Alertness on present units. Tech improvements to communication infrastructures such as phone lines, courier systems, mail etc... should also grant modifiers to concealment of enemies when inside your city radii.
11) I really wanted to work in an ambush mission into all this - maybe I can still do that here (an ability for a unit who's hidden from an enemy that has inadvertently moved into the same plot, to attack said enemy. Same plot to Same plot combat... tricky stuff.) I also wanted to create a promo line that would enable Scoundrel types to get enormous benefits when attacking units that individually had no visibility on them. (Backstab)
12) Down the road, weather effects could work into the visibility system as well.
In short, I think you've managed to work out a method that would enable lot of what I wanted to do here and I'd be more than happy to have you go ahead with setting it up. I noticed we seem to already have some of those effects you mentioned such as increased visibility range on towers etc... but you'll see what overlaps as you go I'm sure. Obviously, you had well thought out different methods of being invisible into your workup, something I had not yet fully incorporated into my plan yet... and I think that's right on target.
One other thing that may go hand in hand here and that's 'states of units'. If we made it possible for units to 'phase out' of physical reality or to travel underground, or underwater, that could play into such a visibility system quite a bit and I think your familiarity with these kinds of 'Domain/visibility' matters could be helpful in establishing those kinds of possibilities as well.
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