So, I have a theory for Fallout 4.
I know we’re all disappointed about how the storyline with Kellog being part of Nick’s mind never went anywhere, but I think we can assume that Bethesda has something planned for a later dlc. I have a little theory on where said dlc could go. I don’t know if it would actually go in this direction, but I think it’d be an amazing route to take.
In it, Nick starts to become more and more fearful that Kellog might still take over him. You both decide to have Doctor Amari do a few tests on him, just to check around to see if his consciousness is still in there. You leave him there and have to wait a day or two, then come check with him again.
You come in again, and find that he’s gone. Amari says that he snuck out, and both of you assume the worst. Kellog has taken control. It’s up to you now to hunt down Nick and either help regain control… or take him down so he can’t hurt anyone.
You hunt him across a new dlc area, coming across his handiwork here and there. Entire raider camps wiped out, no survivors. Here and there are settlements/towns where he’s been spotted, though you question why he didn’t kill them too, and assume Kellog must be planning something. After all, he must know you’re after him.
At last, you track him down to some sort of bunker. You fight through… I guess robots and turrets or something. Just something guarding the bunker. You get to the control panel for the door, and of course it’s locked. However, you see something that shocks you.
It’s rigged so that it can’t be opened from the inside. He’s trapped himself in there.
You go inside, and just find… Nick. Not Kellog. Not Kellog controlling Nick’s body. Just Nick.
And he’s scared. He’s scared that something might happen. He’s scared that he might lose control. So he ran away and locked himself up so that he wouldn’t be able to hurt anyone. He begs you to just end it, just kill him so you can both be sure that Kellog will never get a chance to hurt anyone ever again.
Of course, it can be assumed that you can pass a speech check to turn him around, and both of you go back out into the Commonwealth, worried for the future, but Nick knows that if something ever happens, you’ll be there. He has faith that you’ll be able to stop him…
It probably won’t do this, like, at all, but I think it’s just a great idea for what they could end up doing with the obvious tease of Kellog being inside Nick.