When I first started to write The Obolonk Murders, I had no plan and no idea it would turn into three books. At this point, I knew I really needed to finish up already. One thing Untrustworthy has proven, over and over again, is the value of an outline.
In the meantime, the best thing about the Obolonks is the world building. It is potentially the best-built world I have ever created. Hence the sequels. There's plenty of room in this universe.