The change is that AIs don't evaluate your relationship in like/dislike, they evaluate it in what will get me what I want.
This is where I think espionage could be put in. If you can (with sufficient espionage..not a whole separate thing like BTS, but something you could pour money into) look at the other empire, see what is in their cities what they are building, researching, their happiness balance, expenses, etc.
Then you can get an idea of what their "Interests" are, and you know what they want, and you can think of ways to approach them so that 'what they want' and 'what you want' coincide.
That is what diplomacy is about figuring out ways for both parties to get what they want.
The AI doesn't want to 'like you' they want to get real game benefits from you, either by warring v. you and taking your cities/setting you back or trading with you.
That can't be summarized in a 'number' The best you can have is a Diplomatic adviser that acts like an AI does and says.. they look like they are going for territorial expansion, you might want to prepare for war, or your large military is a threat to them, if they had more money in their coffers they might feel more secure.
I am very much along these lines
Diplomacy: causes of interests...