From Ask us anything: Dungeons:
In the previous Elder Scrolls games, we always had the ability to wander into a cave, old fort, or sunken boat to explore, plunder, and pillage. With ESO being an MMO, will we still have the ability to do this in a solo-encounter type situation where we don't run into any other players, or will we be seeing other players and having to wait for enemies/loot to spawn, or will these all only be group encounters? – by Brett S.
The Elder Scrolls Online is set up to be a fluid experience. As you wander the landscape you’ll encounter caves, forts, tombs, dungeons, and etc. Certain rare locales will just be for solo players, and some dungeons are meant for groups made only of people you choose to go in with. Most locales, however, will be accessible by everyone. Some might be too dangerous for you because of your current level, but you can always come back when you are more powerful if you want to complete them solo. We recommend grabbing a friend or grouping up with a stranger if it seems too tough, though, because that’s all part of the fun.
So, while I'm not sure if the mainquestline is forced solo or doable with a team, there are forced solo locations. I am also fairly positive that the answer you seek is under NDA.