Way back in the 90s I did a contract job at MS Research on a project called "V-Worlds" - a world simulator similar to the Doom or Quake engine, but it was browser-based and everything was a script, so changing how the world worked didn't mean you had to restart a server, just change the scripts and new stuff would appear right in front of you.

Anyway the concept of adding accessories to the player's avatar and even having a pet follow you around came up, and I remember there was an involved discussion of how difficult/impossible that would be. The player's avatar was a special object class that represented a user, and didn't have the same capabilities as ordinary objects in the world. I remember asking, "Why isn't the avatar just a world object the player happens to control? Then you could do all kinds of cool stuff like let the player transform into something else just by switching objects, or let another player run your character." Dead silence. I was just a contractor, what did I know?