The Elder Scrolls V: Skyrim

Skyrim takes the choice of class off the player. There are many skills, and each grows through use: whoever sneaks raises Sneak, whoever stands at the forge raises Smithing. A role isn't assigned in advance; it emerges from what the player has been doing, and at each new level a perk is chosen in the tree of the relevant skill. The world is open from the very start and the dungeons aren't locked behind a level threshold, so you can go anywhere almost right away.

Some of the errands are issued by a system that plugs an unvisited point on the map and a suitable client into a template, so the quests never run out. Dragon shouts are assembled from words on walls and are brought to life with the souls of slain dragons, meaning the hero's development isn't governed by level alone. The longest-lasting consequence lies outside the game itself: Bethesda released its editor, the same one used to make Skyrim, and opened it to anyone who wanted it: since then players have been fixing bugs, translating the text, changing the look of the world, adding weapons, whole regions and rules that weren't in the original. Re-releases have come out more than once and on different devices, but it's this accumulated layer that keeps the game alive.