Visually the Nebulas seem to be a logical heir and I suppose they were trying to recreate the pairing of the Connie and the Miranda, which I believe were very close to one another in most capacities, but the Nebula just wasn't designed to fill the role that the Mirandas seem to have acquired in the fleet. They're nearly at the level of a Galaxy, which is just unnecessary for a lot of fleet tasks. No, I'd say the logical heir would have to be something of a similar size and mission capability but with significant tech upgrades. I suppose something like the New Orleans or the Norways and Sabers might have been likely candidates but, for whatever reason, they weren't superior enough to warrant completely phasing out the older Mirandas. The Intrepids would make sense too, a bit farther down the timeline. None seems to have pushed them out as workhorses, though, although I imagine they've been gradually pushing the Mirandas into more background roles.