I did some digging, and they do share a voice actor.

I wonder if it has anything to do with the ending. If Armin really is the one left(Which is reasonable considering he's the strongest and smartest Titan there is for now), or is it just because he's the type of character that recites or writes diaries/journals, or has deep inner monolouges him himself. Maybe it could, or could not. I'm also wondering if it's accurate as it is. Maybe it could be Armin in the present writing his journal between the events happening and reciting/reading it as such. Considering how the narration started with something like "To you, in 2000 years" or something. Since the popular belief in Eldia was that they were in an apocalyptic world and so it was after many years from the original peaceful civilization. If Armin was Narrating from the end, he wouldn't mistake that, unless he's obsessed to letting his audience feel the drama in the story. My bet is him writing a journal in the present itself. Between the times they are alive with no titans around.

A simple personal theory that I'd not want to turn out true since it's pretty boring one: the story will probably end in something apocalyptic (if not for the world, then at least for Eldians), so Armin isn't so much as narrating the story but sending his memories of it to Ymir Fritz in the past (maybe that is what 'to you, in 2000 years' refers to), so she can see what her playing with forces she can't comprehend will bring.

That implies Armin inherited the entire set of Titans to achieve that, and Ymir will not touch whatever turned her into Titan after seeing the memories. If so, the timeline changes and 'you're free' refers to Eldians as a whole, since, honestly, I don't see any other kind of 'happy ending' achieved in this world, it's either Eldians utterly eradicated or history altered so they never were able to turn into Titans at all (and the world is unaware, so they won't have a reason to murder them for 'sins of the past').

Wow, that might just be true. Considering that Titans do have somethimg that sends back info back in time, to what Eren is possibly having. Maybe Armin found a way to master it's method and send back info? Or probably Hange along with him found something out. Or as you said, maybe he got all the Titans together, since I don't think Eren actually has the heart to absorb Armin.

Or, maybe, it's just one pathetic theory I have. Maybe if all the Titan users along with Historia, the royal blood held hands together, they could possibly posses the power of Ymir? This is because of how Eren managed to control all the Pure Titans around him just by a brief touch from a pure Titan that had Royal Blood. Maybe just holding hands will make them connect their powers?

And about Ymir. Can Ymir be contacted before she inherits her Titan powesr? I think she can, because somehow Eren was contacted by something when he was a kid, just the day the whole titan attack began. And speaking of communication through time, could Zeke be actually told to do all this for the world by these messages and could it be that's what also made Eren change? Since Armin or him or Zeke in the future sent back something so horrifying that he wants to change it?

Anyways, now to Ymir. If she does get the clear discription of the message Armin gives her, how much do you think it would change? Maybe She decides to save the world. Or maybe she decides to do it maybe because she doesn't understand the message(Maybe she got it just like Eren got his as a kid, it could all just be thought as a nightmare too) Or maybe she doesn't care for it. Or maybe that's how it all happened. Like, why not it just be those time travel fictions where nothing changes even when time is tampered with, since it's all forms as a loop that originally consisted of the time tampering. Basically Eren being warned of the Titan attack, could have resulted him in saving his mother or warning the citizens, but he couldn't understand it as he was kid and it was sent to him in a dream, just like Ymir would get it.

I'll have to say, I watched many shows and many books of fiction, but this is one of the best I've seen. Like all the possibilities with the ending and each being possible.

More like Krueger mentioned all Eldians are connected via 'paths', transcending space and time. If that's how it is, Ymir as the Original Titan (i.e. possessing all 9) might be able to exert concious control of the ability to use those paths, unlike every 'incomplete' shifter that came after her that can only access it to a degree (with the Founding Titan being the most prominent user). So, maybe the simple act of rejoining the Nine can allow the user to do something similar, and if there is anyone that can master it, it's either Armin or Zeke... But the latter will most likely die before it will happen and no one will trust him (or Eren, for that matter) with this kind of power in their right mind.

Armin over Eren is also a wise choice because, if Eren wasn't lying previous chapter about memories shaping the personality, Eren already has a clusterf*ck of memroies in his own head clashing (fully his, fully Grisha's, some Freida's via Grisha and some Mrs Tybur's). So, Eren chomping on everyone else but Armin will most likely drive the former fully insane, but since Armin and Eren aren't blood related, Armin will inherit only fragments of Eren's memories, so he will be sane enough to try and control it.

Also, comparing it to Eren's little 'vision' back in the beginning is a little incorrect, since he wasn't even a Titan at that point, only Eldian, so it might be more or less subconcious manifestation of someone looking through his memroies (in case of that theory: Ymir herself). So, with both Armin and Ymir having full control, she will be able to see and undestand things clearly. Maybe being the Original Titan is indeed something akin to being 'a God' and having control over the 'paths'. That means, Original Titan Ymir might be the actual creator of the paths, and have the power to shatter them as well. Without those, the Power of the Titans will have nowhere to go and will die with her. Or she can be reached before she did... whatever it is she did while she was human and it was all a cautionary tale that no person should have this kind of power. Pure speculation, obviously, but I do think a creature like that is the state of mind, perception and comprehension beyond what a simple human can achieve, so simply physical contact will not suffice.

All in all, it seems like the conflict currently draws more into 'Metahuman vs human' territory, with Eldians being hated mainly because the Power of the Titans exists. So, the solution must be, either Eldians are eradicated (Marley way), all other nations enslaved or eradiacted ('New Eldian Empire' repeating history), or Eldians lose the Titan powers altogether. Unfortunately, that'll make them an even easier target for the world that hates them, so the world must forget as well.

So, I think power of the Titans being removed from history, while cheapening the development is the only true 'good' ending the story can reach and the ultimate form of freedom from the Titans Eldians could hope for (ironic, I know).

Don't you think it's more logical that for a happy ending, they would get rid of the power of the titans by somehow 'giving it back' to wherever it came from? Perhaps by someone collecting all 9 titans and then allowing the devil to eat them. Or the devil eating them individually, but that's a bit less dramatic.

Maybe that was actually the original deal Ymir made with the devil now that I think about it. Maybe she was supposed to give the power of the titans back, but refused and was cursed with the 13 year thing as punishment.

• • SPOILER ALERT** u know, I have my own theory of the ending,, just like what u guys mentioned,, there are plenty of possibilities,, mine is this, Armin is the narrator, and it turned out that he is storytelling the history to his children(after married with Annie)..so, when it is about to finish, someone knocking the door and when he opened it, he saw Eren, Mikasa, and their children come for a visit.

Next - how the story ends..as u guys know, the latest chapter shows that eren have the founding titan power and currently do the rumbling (of coz for the sake of his friends and island).. however, if u noticed, Ymir is there during the destruction and actually showing Eren a slightly future vision if he insists with the idea of destroying the world (in other words, Eren didn't even commence the massacre). After Eren have seen the future, he refuses to continue hence return back the power to Ymir. Bear in mind that, Eren did say that he wants this to end not only for Eldians sake but for Ymir as well (She has been stuck alone for 2,000 years!! and waiting for someone to help her). 

My logic behind this is, if Ymir able to lend her power to Eren, it is possible for her to take it back as Eren requested. Also remember, there is a global fleet or army waiting to stop the rumbling, as Eren returned the power - all the titan started to vaporize and this concurrently shows to the world that titans are no longer posing a threat since all of it have "disappear". Currently, Eren's friends are on their way to stop him and this is the perfect time for them to encourage Eren on his decision. Eventually, with the presence of two survey corps "brains" (Hange and Armin) peace could be achieved at that moment as they will take this opportunity to tell the world about everything and how Eldians are no longer the threat to humanity.

Finally, the ending for the main characters - 

Armin will end up with Annie - (as they are starting to develop their relationship). He started to show his interest during the 4 years time skip (even I believe this is all started with Bertholdt's memory of Annie). Soon Armin finds himself that he actually did have feelings for Annie. Also remember, in season 1 - Annie frustrated when she finds out about how Armin looks on her as the female titan. 

Eren will end up with Mikasa - (after everything settled, Eren now have a peaceful mind and can finally return the kiss or tell Mikasa how he actually feels for her - noted that Eren shares the same special feelings for Mikasa since the day he wrapped that scarf) but is always annoyed with how Mikasa treated him as a little brother or son - this reminds him of his mother. This is further evident when he and Zeke were traveling through memories, and he staring and that moment with a sorrowful face. Eren also asked Mikasa about her true feelings for him before infiltrating Liberio. He always wanted to tell Mikasa about his true feelings but was burdened with a lot of responsibilities. 

Historia - many believe that Eren is the father, however, in my opinion, he is not. Why? Simply because he only loves Mikasa and there is no way that he will leave her alone just like that. Eren is a responsible and caring guy. Leyena (Zeke's follower) is the one that arranges her pregnancy as she doesn't want Zeke to be eaten by Historia since she believes in Zeke's plan (Euthanizing Eldians) and post-event she will be the first queen to rule the nation. It also possible that she actually faking her pregnancy (this I am not sure though)

Jean - He will end up being one of the commanders or captains in the military (since he is showed to lead underground operation during their assault in Liberio). He will end up with another Asian blood girl (just like Mikasa) which he met post the event. Bear in mind that Paradis already made an alliance with Hizuru nation (Mikasa's mother origin) and this will make them a diplomatic collaboration that will bring people to migrate to Paradis (already a nation after the event). We knew that Jean has a thing about Asian girls.

Hange and Levi - both of them will end up in the military or in politics. I don't have an idea for them - but I know they will play a big role in the coming years of Paradis rise as a nation. 

Conny - he will always be him, and maybe end up with a girl he met or having a close relationship with Sasha's family. 

Ymir - when Eren returns the power to her, she is finally free from her years of slavery to the royal blood generation. Keep in mind that Ymir's main task is to build the titan whenever a power shifter change into one. Also, she is in charge of healing those inheritors' wounds. I sincerely pity her, just imagine stuck in there alone for 2,000 years. The reason why she couldn't move on to the afterlife is that the power keeps been passing from a generation to another which resulting her being stuck in that realm of time. If u noticed, the first "path" is only three (which after her daughter - Maria, Rose, and Sina) were forced by that idiot king to cannibalize their own mother.  This makes Ymir's soul being chained to it forever as long as the power been passing through. She finally free from the realm and move on to the afterlife, may she rest in peace. 

That's all guys, thanks for those who read to the end. Peace everyone!