If only it weren't for realm mergers being a no-no, I like the idea of being able to keep the Duchy of Koolaris, or rename it Drenga, and have the Guild with two duchies divided by the river. Though I guess that could also be properly made by the ruler of the new Assassins and the margrave of DC.
In all honesty though, I moreso like the idea of just letting the north east be to do with what they have now. Not like they can recruit to any meaningful extent, but they can already do the assassination bit. The only thing they can't do in relation to that is cash bonds, so bounties would be more of a guild thing anyways.... Return of the Assassin's Guild.