TRN One of the gang
The elder council (Has a legion faction that rewards you for patrolling to cities, killing rat infestations and stuff like that.)
Imperial legion faction mod. (Allows you to join them and do about a dozen quests)
As for what you asked for specifically.... I remember a mod called Raiders of Cyrodiil that added a bunch of NPCs to the game and every once in a while during the week some evil factions would attack the town militia set up outside towns.
It was a little buggy so I eventually got rid of it... haven't seen any other mods like that, probably because most modders are afraid of being dirty, having important NPCs accidentally die, and stuff like that.