Well, gameplay wise, alot. But most of it has been said.
So, plot-wise. Akavir. We've heard so much of it, it's caused many discussions in the Lore circles (I consider myself a bit of a TES lore buff). And this seems the PERFECT time for them. No Emperor, Tamriel in chaos and possible anarchy, ravished by the Daedra. I think it's time the Akaviri showed up and tried to invade again. It would introduce a whole new aspect to the beastiary with the vampire snake-men and whatever horrors they might bring along, to say nothing of Akaviri weapons and armor beyond the katana. You could introduce something that felt truly alien like the island of Vvardenfell did, and it would open the door for true epicness. Imagine if they improved the combat and set up a scene or two in the main quest with large battles between the forces of Tamriel and Akavir. Or if you don't want to chronicle the actual war, let us out of whatever prison we're in now after the war is lost and have us try to overthrow the new Potentate. Whatever the case, this setting is rife with new things for the series, from new creatures and equipment to foreign Akaviri magics being put into the Tamrielic spellweaving society.