Oblivion and Skyrim had Roman and Norse influences, respectively, while Morrowind was more Babylonian and Assyrian, as well as various other real-world analogues. Cyrodiil thus felt a little too familiar, as so many RPGs draw from that well as much as the works of Tolkien and the western mythology that predated LOTR. Dragons and the Nordic feel of Skyrim were fine - and the updated graphics and storyline were acceptable - but the monsters, characters, and storyline were more unique in Morrowind, even if it made for a much bleaker (blight, corprus, constant cliff racer attacks) world.
The Nerevarine quests felt a bit more rewarding to me than being Dragonborn in Skyrim, or closing the gates in Oblivion. But the big advantage Morrowind had was how much mythology was put into and crafted for the game. I remember reading somewhere a comparison of how many in-game books were made just for Morrowind. Oblivion and Skyrim felt a bit dumbed-down next to Morrowind. Granted, it probably increased the popularity and accessibility of the latest Elder Scrolls game, but I'm not positive about the upcoming MMORPG. I hate MMORPGs, they are just mindless.
I don't play video games that much anymore, but I still appreciate good stories. If the next Elder Scrolls game isn't at least as good as Skyrim or Oblivion, I'll probably skip it.
As has been mentioned, they definitely need more voice actors. A secondary concern for me after the storyline and lore. It would be cool to see the next game take place outside of Tamriel.