Morrowind is great, you can't really call it a themepark though, considering it is a singleplayer game in the first place.

EVE Online, Star Wars Galaxies and all the big "do whatever you want" games, the ones that don't confine you to a bunch of preset activities and let you engage in whatever emergent gameplay instead of banning a bunch of people and patching whatever they were doing because it wasn't entirely planned out during…

Just TESO and a few other large releases in the last few years that people have been terming "sandboxes".

It isn't a sandbox though, its a themepark, you are given a bunch of
stuff you can do and very little opportunity to create your own content,
you are not making a sandcastle or smashing someone elses, you are
going down the big slide while someone tells you what a good job you have done, rather than going out and…