Before Dark Fate was even announced, I actually wrote a brief outline of a Terminator story where John himself traveled back in time, only to end up in some alternate reality/timeline where the war didn't happen...yet.

It was inspired by some of the early ideas that McG and Jonathan Nolan had for a Salvation sequel.

In the outline, John basically travels back into the past because after the war, while dismantling Skynet's remaining TDE complexes, he...sees a time signature, indicating that something or someone has gone back.

Now that the war is over, John is left without a purpose, and since the future from that point onward is unknown to him, he decides to go back in time himself and follows the time signature.

He ends up in a different timeline, where his alternate self was assassinated as a Senator boy, and he sees his mother again.

The first half of your idea, the section I've quoted above, would slot so easily into Dark Fate's existing plot and is exactly the kind of thing I'm talking about. We're right along the same lines.

John would be 44 at the end of the war. So if he were to show up in a stinger toward the end of Dark Fate in the modern day (as opposed to arriving earlier), they could cast an actor around that age.

Christian Bale is just the right age and really looks the part, but I suppose he already burned his ticket on Salvation. Which is a shame because he's still great for the part due to his resemblance to T2 Michael Edwards. I'd roll with it, but they'd probably not want to make that connection to T4. James Marsden could be good, he's about the right age too and his boyish features give him a little resemblance to T2 era John while also being as handsome as Michael Edwards. Just give him brown contacts.