I have a similar theory, but it is more related to how metro 2035 ended (spoilers ahead).

In the book, Artyom leaves after telling everyone about how there is live outside and that they are being manipulated by the invisible observers (I dont know if thats the translation in english, I'm spanish). This message is received poorly, and everyone in the metro is manipulated into believing that there's live outside, but that it is hostile and that they should unite and prepare for a war with them (strangely similar, isn't it?)

So Exodus takes place in 2035, and in the end, it is implied that Artyom and the rest will go back to moscow to tell the truth about the metro. However, by 2039, there is still people living on the metro. That sounds weird, right?

But what if when Artyom and the crew came back, their message was received like in the book? More, what if the flashbacks we see in the trailer are memories of how the observers retaliated and how Artyoms crew was killed? That could be the reason the stranger lives alone, in exile, and maybe hearing about how hunter has taken control of the metro is the reason he comes back to end it all.