Something I've run across in many of my games is that towards the Industrial and Contemporary eras the leading 2 AI players will vassalize the remaining AI players. This becomes frustrating when I've decided to run with cultures or wonders that give a bonus per alliance and the only unvassalized empire hate me because, well, the 2nd and 3rd place AI almost always hate me.

Something relatively simple that might help with this is if another player's vassalization breaks one of your alliances, it generates a grievance against the liege. This would allow the player to essentially declare a war of liberation to free the AI player and potentially reinstate the alliance. Ideally, freeing a vassalized AI player would add a positive buff to their relationship to you.