So this theory leans more to the anime than to the manga. So the narrator of anime in SnK sounds like a feminen voice that's more deep. In the OVA where the 104th training group split into groups with a set captain, Armin was narrating the entire thing. In the end of that OVA they pretty much said Armin was talking the entire time.
Armin's voice when he narrated the OVA sounded different then how he normally talks, lower voice like he's grown up and developed a lower pitched voice. That Narrator sounds exactly like the regular SnK narrator. What if the regular narrator is actually Armin but when he grows up?
Maybe he's the very few people that survived in the end and he's telling people that were supposedly in a place with no Titans. Or hes telling the new generation about the Titans existence?
Or or he wrote down everything that happened in a book and is now twilling everyone what Eren and the Scout Regiment has done.
I might add more to this theory later.