I'm planning to run both the classic Eternal Boundary and Harbinger House adventures in my new Planescape campaign (much thanks to Hymneth for the conversions). As I'm certain others have done before me, I'm looking for ways to link the two adventures together into a larger story arc. I'd like to hear how you (or your DM) went about doing so.
Also, I've never really cared for the Illuminated being the antagonists of the Eternal Boundary; they always struck me as a poor choice since there isn't much written about the group and little tying them to the City of Doors. It seems to me that any evil and manipulative group with an interest in influencing faction politics could be swapped easily for the Illuminated. Some groups I imagined in their place were Baatezu from the Temple of the Lords of the Nine in the Lady's Ward, an ambitious swarm of cranium rats (perhaps the Us) and/or Illithids, a group of Yuguloths led by Shemeska the Marauder, etc. I'd be curious to hear your ideas and opinions on the subject. Thanks.
I always thought that an anarchist cell would work well. It's the stated goal of the anarchists to bring down the other factions, so that's motivation enough for a group.