I've thought about that - and it's not that big a jump to have a GUI that allows a user to specify a win condition (and locations to traverse, other conditions etc). Then, the user can share it online. I haven't played GTA V online, but I think it does something like that.
But actually, I don't want those sort of win-conditions for this particular game, not even DIY stats. It's just an immersive simulation. Guess I'm just not too sure it WILL be immersive or engaging - even for me to play! Just have to see, I guess.
Taking your lead, I suppose if a movie can be immersive, why not a "game" which has no win-condition? But the moment I say that, I immediately realize that movies do have "win-conditions" - just of the protagonist, not the viewer/player. Goals, stakes, motivation, obstacles are all important narrative components.
Can you have an immersive movie without narrative? Sort of maybe, some experimental/arthouse, but they aren't very popular (and arguably do have some kind of narrative).
You're probably right about definitions, but they do inevitably lead to the components of a game/non-game, and whether they are engaging, and what makes them engaging... and they're easy to ask.