Now that characters in world info are a game feature, adding a relationship category wouldn't be too difficult. However, I'd like to see a feature where the AI can recognize relationships and it affect the actions of the characters.
Very simplistic example ~
Rose gifts Joe a stick (neutral relationship)
Rose gifts Joe a diamond (positive relationship)
Rose gifts Joe a punch in the face (negative relationship)
Unless set otherwise, all relationships start with a neutral value. In the world info, users can create relationship types and set it to one of the three (or more) values.
Lover - positive relationship
I'd like to see a lot more detailed relationship features in the future, but I think this would be a good start in implementing it. Eventually, I'd like the AI to be able recognize a "lover" type relationship as two people who would treat each other affectionately. On the other hand, if they were friends, the AI wouldn't cause actions like kissing since it would be abnormal for normal friends to do so.
If Rose (Rose is the player) & Joe's relationship = lovers then Joe can kiss Rose
If Rose & Joe's relationship = friends then Joe can not kiss Rose (can but will not)
I do think these relationships should be able to evolve, perhaps by assigning points to relationships. +1 point over spending a certain amount of time together or -1 point for being apart for a long time. Player actions can affect their own relationships as well as those around them. If they were to hit someone, their relationship with then would go down. If they were to talk bad about someone with someone they're close to, the other person's relationship with the person the Player talks bad about would go down and vice versa.
Rose helps Joe off the ground (+1 friendship)
Rose flirts with Joe (+1 Romance)
Rose hits Joe (-1 friendship)
Rose tells Joe something bad about Mary (IF joe's relationship with Rose = friend (or something like 10 friendship points) THEN Joe's relationship goes down with Mary (Joe -1 friendship Mary)
IF Joe's relationship with Rose is neutral, THEN Joe's relationship with Mary stays the same.
I have a lot more ideas for this, but I'll leave it here for now.