The faction/reputation system is kinda half-assed implemented already, kinda.
There is a community value that indicates which NPCs help each other, with community -1 I believe being neutral (non-hostile unless attacked). I think this value also determines which ones attack each other as well.
I dunno how much work it would be to have a table tie into this community id number and tie that to each character (probably added to the table the character values are apart of), and then have events raise or lower each value. Where if you are above or below a certain value determines hostility.
The issue is community IDs are kinda wonky right now. There isn’t much rhyme or reason except when they don’t want certain NPCs attacking each other. So much of the work would be to logically reassign these values.
But as all this is tied to the character (for example, if I beat up Blackhand with my character, your character shouldn’t lose faction with them). This needs a bit of thought on how to implement. Also thought needs to be had into how mods will tie in. Many custom NPCs and stuff with Custom maps utilize community IDs already. But none of them are saved as part of the character tables currently.
Mods would welcome this I’m sure, especially those with custom factions. But its not an easy addition. But thankfully I don’t think its crazy hard either. Some of the work is kinda done.