If you haven't been keeping tabs on Mojang's Lead Developer stating that "no NEW dimensions are going to be in the making anytime in the near future." Though they are not completely doing away with any possibilities, they simply do not have the energy, time, or money to put into a new dimension project, however they will do so with updating a pre-existing one such as The End.
Yes, after the dragon is defeated it is technically the end of the story of MineCraft. This being said, is not a justifiable means to why such a hard to get to dimension has to be so bland and boring. For example... We see Endermen 100% of the time we are in both the overworld and nether and then nothing but Endermen when we arrive at The End. The lack of mob variety is sad for such a mystical looking place. If you were to even watch a video of BetterEnd Mod you could see what possibilities are achievable with such a dimension!