They were making it up as they went. The Defiant was created when the original idea of the majority of the action being on the space station was discovered to be too limiting.
Actually I seem to recall some interviews published before or during the first season where they hinted that there were plans to eventually add a larger starship. But I think they were talking in terms of using it as a vehicle to do more exploration-based stories in the classic Trek vein, so I doubt they were thinking of something like the Defiant
at that stage.
But no, there was no master plan from the beginning. The original showrunner, the guy in charge of the creative process, was Michael Piller, but after season 2 he shifted his attention to Voyager
, and then Ira Steven Behr took over as showrunner for the rest of the series. Also, after season 2, Ron Moore and Rene Echevarria moved over from TNG to join the staff. So a lot of the key decision-makers in the final 5 years of the series weren't in those same roles for the first 2.
Plus there were decisions that came from the studio, like adding Worf to the cast in order to boost ratings, and shifting the focus to a Klingon arc and away from the Dominion stuff for a while. The studio was also constantly pressuring them to avoid the Bajoran religion stuff, though evidently that pressure relented over time.