Not quite true; Tamriel is everything that is showed on the mapt you linked. But there are other lands, although they have never been included in the games except in lore.
true that. it's like when people thought that the world was flat. nothing (or very very little) is known about what there is over the edge/ocean.
Castle crasher's world! Where peasents can save the day, where you can die and ressurect, where charecters look cool, where giant moose-frogs are used to store weapons, and where behemoth studies is a council of gods!
runescape is a good one you get immersed in the gameplay and getting your levels higher to make more money to buy better equipment so your character can do more its a life
my favorite virtual world would probably be pokemon? do you mean like a world that doesn't exist? that's made up? if so then maybe soul eater or Kingdom hearts....