Season 1, 2 & 3 all had fantastic premises I would have loved 7 seasons of but were all unrelated and concluded within a season.
Season 4 actually demonstrates the missed opportunity, they deal with the fall out of season 3
For example if you think of the scene set in "A Vulkan Hello", you would have ended up with an Action focussed version of DS9.
You didn't need a spore drive, Jason Isaacs could have stayed the same and we could still have watched scientists struggle to become soliders with the war causing the type of fall out we see in Season 4.