Well firstly, for that to work, a lot of fundamental concepts have to be changed.
1) The AI cannot declare war simply because it wants to, and should declare war when the relationship is bad (similar to BNW system) of course, we can separate the AI into "good" and "evil" AI personalities as in BNW too... (Theodora VS Attilla for example) with evil AI being harder to befriend to begin with. AI should follow their personalities rather than just be total jerks whenever they feel like it so you can at least predict what they are going to do (for people who want that, there's then random personalities option in civ V... I don't mind having a few backstabbing villains in the cast, but let it at least be obvious who's who). I've been betrayed by Tomyris, of all people, too many times more than I can count for example, had high level alliance, then suddenly won't accept friendship despite being at +50 or so... dozens of turns later, she declares surprise war. Same with Gandhi who simply accepts a joint war and refuses friendship after eras of alliance.
2) War must be VERY punishing, especially to the offender (give defender more advantages) and at best it should be a lose-lose situation for the aggressor (when he declares war he achieves his goal of slowing down/destroying his target--this should come at a cost of giving other, uninvolved AI chances to overtake him); the happiness penalty system and courthouses, as well as buildings breaking in BNW does this fine... war should not have a separate side reward to it aside from inflicting a minus to the target civ who you feel is winning. Encourage overtaking your opponent by outrunning them rather than just kicking them in the shins.
3) Warmonger penalty must be like in BNW. You are to become public enemy number 1. Let every surprise war after ancient era trigger an emergency. Make AI offer HORRIBLE trades when they hate you (makes no sense that having -100 from warmonger penalty, they still want one of my luxes for 20 gpt... while being allied with them they offer only 5 gpt)
4) Bring back non-war ways of retaliating (world congress and UN in BNW).
In terms of diplomacy, BNW does it way better. Civ VI has always been a joke with this regard that I do not bother with diplomacy other than the super early game as an option not to get instantly killed.
