Weighted randomization (using the default value more often than other values) and based on the game-state. Morale seems like the most logical value to use.
Weighted randomization surplus which depends on morale is good, but also reverse scenarios are acceptable: when units that are supposed to be zealots, start to hide for a random amount of turns and then became zealots once again, coming out of shitholes and dark places of any kind.
Well, based on my perception of BAI's previous versions, it usually makes it hard to proceed without casualities within player's troops, setting up ambushes quite good, but rarely relying on reserved TU's for reaction shots. From player's perspective, this lowers the most consistent part of the joy (a gambling within a game), making it rather a straight-forward chess game, than a a sort of rogue-like role playing game.
I can't comment current BAI, as still want the thing to happen: be it possible to rewrite all RUL aggro settings manually. Now it comes much closer to that.
Remember, that aggressiveness level written in OXCE RUL file has nothing in common to be compared with BAI aggressiveness.
So. There is HARDLY an option to
"choosing option 4 means aggressiveness is inherited from unit-aggression" at the current state of BAI-mod (XPZ/XCF) compliance, because units then will behave unnaturally, and most likely push themselves to death, even weakest ones that should otherwise passively run around in vanilla. And, btw, they run around so to make mission more mild/narrow in terms of shooting density (such balanced, that's it). So player don't have to deal with all 50-100 troops simultaneously turn 2.
And one more thing. There's invisible units on player's side too. By invisible I mean really invisible, not CAMO. Last playthrough I had one catgirl making beef out of whole BAI cruiser because they couldn't do anything regarding it. Invis = 5, range = 20.
Of course, I cancelled reaction shots my turn. But vanilla has sniper-spotter. What can BAI make in this case?