Add an option for players to give a settlement to "Rogue Armies", transforming them into standard factions and giving a large diplomacy bonus to the gifting faction.
Add an option for players to pay rogue armies to leave player/player ally land.
In my current campaign, I'm being raided by Tor Ethilis Refugees which are a "rogue army" but High Elves. They are raiding me after we signed a non-aggression pact which is annoying and, I think, rather unlike High Elves, but that issue is for another thread. I signed the pact because, even though my best army was nearby, I have other wars to fight and I like High Elves. Besides, it says "refugees" in the title of their name so I feel badly for them...and shouldn't they be doing things like asking me for gold and land rather than raiding me? Anyway, I was thinking about how to get them to stop raiding me without taking a huge hit to my reliability because we signed a NA pact like 3 turns ago...and I thought it would be cool if I could give them a settlement or two and have them turn into a new faction. I would happily settle them in Estalia--I'm playing as Clan Angrund and the Beastmen faction in Estalia declared war on me so now I've destroyed them and colonized the whole country, but I don't really want it. Having a friendly ally or vassal there, at the back of my territory, would be a neat alternative to directly controlling it, and it would be a creative way to get rid of the rogue army which is now raiding my lands.