I also agree with moneal regarding the oddity of not being able to become friends with enemies; that is a rather crucial human behavior of societies.
I've already started changing this, which has let to the questions:
when should a neutral become hostile?
when should a hostile become neutral?
now instead of friendlies and hostiles, 90% are neutrals (friendlies were already basically neutrals), and 10% are hostiles (bushwhackers). there are also plans to add thieves and slavers. bushwhackers try to kill you for your stuff. thieves just shake you down. slavers attack to subdue and attempt to capture you.
Perhaps model it on venn diagrams rather than raw individual percentages.
Let's say the static value is broken into 0-25 = enemy (category 5/1), 26-45=unfavorable/distrust (category 4/2), 46-65=neutral (category 3/3), 66-85=favorable/trusting (category 2/4), 86-100=friend/ally (category 1/5)
But then let's say that you are 75 with group A, but group A is 86 with group B, and you are 25 with group B.
So the balance of your relationship with group A would be the crossing point between B and A to you...perhaps that formula would be something like group A rating less (group B rating to group A), so perhaps some interplay can occur here where say, if you use group A to talk to group B, group A's rating on your behalf is 36 (difference between you to group A and you to group B, less Group A's rating to Group B), and perhaps this inversely could impact your relation to group A at times...
Focus on the first number of each category for a moment e.g. category 1/5, only think of this as "category 1", for the moment.
say the standard rate is 1 point per friend point when they are category 1 range and 5 points per friend point if they are category 5 (0-25) range to you.
But in this environment, it could get more complicated...the rate of friend point gain on that 75 that you have with A is dependent somewhat on your relation to group B...meaning, it costs more; so, perhaps half of the your relationship aggregate of friends of group A (in this example, that's just one other entity, group B) is added to the cost to group A gain in friendship for you.
So, A, in this example, would normally cost you 2 points per point of friendship, but due to group B in category 5, the cost rate is 4.5 per friendship point gain for group A for you.
Now, flip to focusing on the second number in category 1/5, so that we are looking at "category 5".
Equally, it works in your favor in friendship point earning to group B, so if there is a favorable friend to you that is a friend to an enemy of you, then cost is lessened by the difference in sort of reverse at half the category rating of the difference of group A's friend rating to B less your rating to group B.
So Group A's friend rating to group B is 86, and your friend rating to B is 25, so the final rating is 61, and 61 is category 3/3 (neutral).
Half of 3 is 1.5, so now we take the cost for group B and less it by 1.5 in your favor due to your friendship of their friend.
So the cost per friendship point to group B would normally be 5 points, but with the aid, that becomes 3.5.
So a friend weighs a bit in your favor to making a friend of an enemy, but an enemy of yours that is a friend of one of your friends weighs even more.
So...this encourages to "play nice", or rather...be political as hell and get those hands greased up, or you can just pay the extra for those whom you do like and continue to tell their friends to piss off.
This is only a conceptual model to express the idea; and is entirely capable of being scaled to whatever fits best, and can be compounded by many factors beyond these simple starting variables.