He's a ghoul.

Or... The real you!

I don't really think it would be unlikely at all. Shaun did make a synth copy of himself after all. It's only reason for existence was just one of his whims. The creation of other copies, then would be just as likely (although he would probably have to guess their personalities).

In the case of the fate of the Institute, the only real difference would be how the DLC is introduced.

If you sided the Institute, it could be you being sent to hunt them down because they escaped. The story can give you another look at the synths as you find these clones of people you once knew trying to survive without any knowledge of the wastes like you and Valentine did.

If you destroyed the Institute, you could have someone Preston tell you about some synths causing trouble somewhere and you have to go solve it.

Seeing as how FO4 was just basically everyone reacting to the synths being sentient, the real purpose of the DLC would be that it shows how the synths could possibly gain sentience and its implications in the post-apocalyptic world. The computers in the Railroad HQ show that some of the synths they memory-wiped became raiders, so it could focus on the moments like that when giving something freedom goes wrong.

For a less synth-focused quest line (and probably, more boring), they could just be used like in that one Mass Effect DLC, where your clone is trying to replace you.

IMO, though, FO4's story was already pretty weak and a DLC based off of it would have to lug that weight with it. Also, I'm pretty sure Bethesda wouldn't see any point in making DLC focused on synths when that was what the main game was about anyway.

TL;DR - I agree, it is unlikely, but I also disagree because I think it's not for the reason you said.

Well, no shame in two intelligent people reaching the same conclusion by different means. But more than this theoretical DLC, i want to see either something like Dead Money again, or something with more underwater. I loved dead money, and i spent two hours wandering around the ocean floor in power armor and eating mirelurk cakes. I was getting nervous that the game would say "OK, you've had you head in a bucket of water for the past two hours, now die horribly and go help another settlement."

Not you. Your synth spouse, raising the question of whether it was really him/her who died in that vault, and not a synth copy.

It has a removable cap. Put that back on and no piss leakage.

Also, that's going to be the least of his problems with the puke ad blood from his wife and puke shit and piss from the baby.

I see no problem with giving someone your pregnancy test.
Proves you ain't lyin' too.

He's alright now

I'm sure he's had worse