I knew some people who put together a demo for a MMORPG that would support people playing the monsters or the players depending on what their particular play style was. You could build your character, have social interactions, level up over time, and put all this attention into your skill tree. Or, you could run around and fuck shit up, potentially as a massive OP badass boss who could mow player characters down, and then you would respawn instantly when you (frequently) got killed.

I don't know of it having been done in exactly the way they wanted to do, although a couple of the games in the Aliens franchise came pretty close. There was also one fascinating mod (I legit can't remember the name) where one player on the "Marines" team was basically playing an RTS in the ship giving orders and the rest of the team was running around playing an FPS against the (also player-controlled) alien team. It was a ton of fun.

I miss the pre-AAA-shitfest era of gaming.