As a test, I am thinking to disconnect Zeus, CoA, CdM (-225%A/-225%D) leaves me with 800%A/600%D in ME. I can see how it affects GE L1-4, probably lose a few units on auto battle. But the real test will be to use GBG opposing army and attrition to see how far I can go, auto then manual battle. This way I do not have to change one thing in my city but I will know how far I can get versus the opponent. Then this becomes useful to determine what city changes I COULD make, replacing some attacking army stats for defending army stats.
Many buildings have 1 A/D for attacking army, like winners plaza, Colossus, Stages of Ages. I think event buildings stats to increase using just attacking army have hit a wall. So, we can get 2 or more A/D on event buildings, they can use defending army.
Buildings currently exist and will probably become more popular.
Example SAJM - same number of tiles
3 stages of ages = 57% A attacking
2 Jolie canals = 48% A attacking, 60% A defending
So, you lose 9% A attacking but gain 60% A defending (gain 6 FP, lose 45 previous era goods daily, the goods loss isn't going to make you a nego champ, 300ish a week).
I am more concerned about the diamond heist going on in L1-4 and no diamonds in L5 where many would be spending to complete negotiations. I have read the concern about new players and GE5. New players wouldn't even consider L5 for a while. The loss of unattached rogues and champs in L1-4 will be a bigger negative to newer players. Others ways to get them (big traz, GBG waterfall prize come to mind) but the little things matter to newer players.
I mentioned making the portraits like BPs, 2x2, additional reward, not the 10% reward. You can also have 1 unique portrait for completing GE5 fighting, separate from the achievement you get from completing L5, most will do via negotiations. Make this portrait a one of a kind.