Multi-player has no place in a single-player story telling RPG like The Elder Scrolls. It just wouldn't fit. You are supposed to be the hero, where all the odds are stacked against you, and you must vanquish evil alone. Your character becomes special because of this, and there is more of an epic feeling in the world. Once you introduce more heroes, that weight is lifted because now you have companions to help shoulder some of it.
To add to this, I'd rather see them spend all their efforts on making a great single-player RPG, that's polished, is feature rich, and has a story that really draws you in, then have them split their efforts in order to make the game work for both multi-player and single-player. If you want a multi-player RPG experience, there are so many others out their specifically designed for multi-player.
As for Fable 2/3, instead of adding in a, in my opinion, pointless multi-player experience, they should have spent the time making the game good. They both went downhill, with Fable 3 at an all time low, from the original.