Two cyborgs have traveled back in time to Fallout 4: one wants to kill you, one wants to protect you. That's the Terminator-inspired idea behind Marked for Termination, a new mod that adds two NPCs to your game when you reach level 10. The baddie, who presumably also looks exactly like Arnie, will hunt you down wherever you are on the map and attack on sight. It cannot be killed, which means you can try to incapacitate it or—probably a better idea—run for the hills.
Thankfully, the other cyborg will stay by your side at all times, and won't override any existing companions. They too cannot be killed, which means they'll always be ready to step in when the time comes. I like the idea of crouching in cover and watching the two machines duke it out until one of them needs to crawl away and repair itself.
To ensure the mod isn't a constant annoyance, the enemy cyborg travels around the map at a walking pace, which means that if you're always moving then you should be able to stay just out of reach. The mod's creator, ToroMontana, says "it's just for fun, mayhem and putting you on constant alert", adding that they may update the relative power of the cyborgs based on feedback.
I'm not sure you could play the whole game with it because constantly skirmishing with an unkillable enemy could get annoying fast, particularly if they're stopping you from getting where you need to be to advance the story. But it does sound like a fun distraction, or even something that could be central to a role-playing run. Now we just need to work out how to make Sarah Connor in the character creator.
Grab the mod from its Nexus page.