I think you're taking the meaning of Protective and Defensive too literally, You wouldn't consider it protective you take back a town, improvements and land that originally belonged to you???
Uhm... probably you're right.

The point it is that Defensive trait to me isn't bed in term of promotions the way it is. I think about a Cho-nu-ku enanched more than what it is, and I think it would be too much.
The Fact the the bonus only works on the defensive is almost useless because you require to build ALOT of units to defend you're many broader tiles unless you're lucky to be blessed with a Peninsula and all those unit are gonna cost you, slowing down your economy.
Use the unit in your city, when you see troops coming bring your units out and defend fortifying what's worth to be defended, and attack when strategically better (enemy on flat, not in stock, ...).
Lowbowmen with CG3 fotified on flat land
6 str + 25% from CG 2&3 + 25 from Fortifying = 9str
Macemen Unpromoted Fortified
8 Str + 25% = 10Str
You forgot the FS and the FSChance from the DrillI promotion. Beside that, Crossbow has +50% against melee units.
And don't forget that what you give to Archery you give it also to GunPowder!
Said that, if the +10% CGII and +15% CGIII bonus I proposed are too low, they can be raised to +20% CGII and +30% CGIII (or even be the same as "in city": +20%CGI +25%CGII +30%CGIII). Whatever option need to be tested to say how "fair" it is.
so improving that won't change people's gameplay.
Protective is fun to play to me. I see what you explain, and I agree that cultural borders of Defensive Empires should be "harder" to invade, and "easier" to defend; but not to the point to break the balance. A promotion line which makes a unit better defending, attacking, agaist melee, agaist mounted, ... , without a weakpoint, seems to me a little too much. Still, I can be wrong, it should be tested; I'm talikng about the idea "on the paper".
I think we agree on what's to emprove (the defence of the "inside the cultural borders" lands), we have yet to agree on how/how much to improve it. Anyway, I find interesting to read your point of wiew.
And I've already Said my 2 cent about about the extra 10% within cultural broaders... it's too generic... I won't a unique bonus that will keep the the trait interesting and different to the others.
I can agree on this. I also like better the enanched CG line option. Lets say now my official position is "Option A"
here's another Idea instead of creating a new CD promotion, how about improving the Drill line, most people generally ignore this promotion line because the Free Strikes seem too random.
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I'm just making this Promotion line more Likable as Drill has kind of been under-apreciated as most Protective players usually just go down to CG 3 and use their defenders for new captured cities.

I tought people loved Drill line with a Protective Leader! I tought nobody liked CG line!
Drill IV has a big reduction of collateral, a great chance to don't get too hurt winning a fight (since the first rounds can't get hurt), and also if not comparable to Combat IV, a quite bigger chance to win a fight agaist a "same streight" unpromoted unit. And with protective that's only 10xp.
Once you don't have to pick Drill I (which worth a lot less than Combat I, I agree), the other in this line are good benefits promotions. The only real weak point are the "Immune to FS" units (the +10% bonus agaist cavalry of Drill IV is there for that reason I guess).
Instead CG is kind of weak: excellent to defend cities, no doubt. But too unflexible: beside defending a city, it's an unpromoted unit.
This makes me also think that you didn't think about the Drill I line as a good counterattack strategy and you think protective units needs one extra attack bonus. Keep in your cities 50% CG and 50% Drill units, 25-50% Longbow and 50-75% Crossbow (later Granateers and Rifleman). Few Pike in case of Mounted units coming. When attacked the CG units will kill the siege units, the Drill units take less collateral and strike back.
I just think that if this could be done on resources as well il would be great, that why I think the CG improvment as something that worth a try. Drill line seems valuable the way it is to mee, especially with a protective leader.