I see what Garnett is getting at with the Dark Souls idea in GTA, but no - keep it out.
Can you imagine trying to finish a mission, like a car chase for example, and someone suddenly invades your mission and runs you off the road. You fail the mission because someone came in and screwed up your game.
NO thank you. If you want to use the sandbox analogy, it would be like building a sandcastle and someone else coming over and kicking it over.