Ivor's plan was the same as the rest of the Fourth Ring ... to destroy Gomorra. That was the mission they were given and that's what they set out to do. Ivor chose to do it in his own way with his own style, which as a servitor of Pestilence, was through disease.
The entire arc was meant to reflect that in various ways. Ultimately, he would use his disease to infect the entire town slowly and destroy Gomorra that way. The Blighted were remnants of that plan kept around in case he would need them later (or to do with as he pleased once the overall mission was over). The explosives were the mechanism for unleashing the Blighted should the need arise.
At the same time that Ivor was infecting and spreading disease via individuals, he was also metaphorically infecting the town as a whole, like Gomorra itself was also getting sick in a way. Paul Durant actually came up with a lot of the parallels between disease/pestilence and Ivor's plan ...
1) Not only was Ivor slowly spreading his disease, but he was spreading lies about how to prevent it and cure it. These lies were based on historically false thinking before the advent of modern disease theory. For example, the idea that miasma, or bad air, was the cause of many diseases during the time period gets spread by Odett and the Sanatorium as well as the creation of the quarantine tents.
2) The Blighted pockets were like "pustules," filling with decay and disease, and the explosions were his way of "popping" them and allowing the decay to ooze out. The card "Open Wound" was named that way for this reason because after the explosions, they were holes/pockets of infection left behind as an infected wound on the town itself.
3) The idea that the cure for Ivor's disease couldn't be a "cure" in the truest sense was the same way. It ended up being a counter-pathogen ... something destructive to the person, but survivable and far MORE destructive to the other disease. Not only did the game's timeline not really allow for a true "vaccination," but it was also indicative of the town itself. If Gomorra is viewed as a life in and of itself, then very few, if any of the outfits could be said to have a true healing effect on it. But when they banded together to face a common threat, it could be said that they were far MORE destructive to the thing presenting the clearest danger to Gomorra. So the heroes at the end, though all part of the conflict that tears at Gomorra's seams, are in essence the "counter-pathogen" that can bring down Ivor. (If that makes sense ... I know the metaphor can get a bit lost sometimes.)
The plan was always for the game to go in thematic cycles, which would play out in as many ways as possible. These were some of our attempts to make that work for the Pestilence arc ... we were getting ready to dive into the War/Conquest arc when they pulled the plug.