It's amazing how many questions Naughty Dog were able to raise with a single 4-minute video wasn't it? Especially as most of it was a music video! Anyway, I've been thinking (probably a bit too hard) about the The Last of Us Part II Reveal Trailer and I have a few thoughts that I'd like to share with you.
WARNING - This blog post will contain spoilers for The Last of Us, Left Behind and the reveal trailer for The Last of Us Part II
The Game's Title[]

Not 'The Last of Us - American Daughter' or 'The Last of Us - Ellie's Vendetta' or anything like that. 'The Last of Us, Part II'. Neil is telling us that this is not a sequel, it is a direct continuation, time skip or no. We are following on from the end of the original game to follow Ellie and Joel's personal and joint journeys onto their next phase, for good or for ill. There will be no new plot arcs or new major characters to act as antagonists; we are going to be following on with the characters, plot and events of the first game.
This is emphasised by the fact that Ellie is sitting by the window that was the background for the main menu screen on the original game. We are being told that the journey to date, from Boston to Salt Lake City, was but the prologue. Everything we have learned and shared with Ellie and Joel so far has just been getting us here - to the beginning of the story.
Joel - Alive or Ghost?[]

Ellie's Dark Angel?
One of the most pervasive internet theories that I've seen arise is that Joel is dead and that Ellie's motivation is to avenge him. This is based on two interpretations of the video:
- When Joel appears in the door of the abandoned house where Ellie has just apparently committed a massacre, he seems to materialise out of an ethereal light;
- Arguably, what surviving human relationship does Ellie have left where loss would drive her to such a murderous (and borderline psychotic) rage?
I have to say that I'm not a fan of these ideas. Whilst I agree that there is a symbolic aspect to Joel appearing out of the light, I don't think it is him descending from Heaven (let's face it, I can't see anyone in Hell having the guts to let him in).
In brute, literal terms, the light difference between the inside and outside of the building would be enough to create a 'dazzle' effect, preventing objects that are outside and too far away from the camera from being visible. Additionally, on a symbolic level, I think that Neil is giving us an early indicator of Joel's role. He's going to be Ellie's light, just as she was his light in the previous game. He knows what it's like on both sides of the moral fence, after all; I can see him wanting to be there to make sure Ellie doesn't stray too far onto the wrong side.
Additionally, whilst losing Joel would make Ellie sad, I can't see it driving her to the level of despair, hate, madness and uncontrolled rage that we see in the video. No, she's been stripped of something far more complete.
Who Are 'Them'?[]

The Firefly logo in the trailer.
Whilst I agree that the presence of the Firefly logo is not, in itself, indicative of anything other than to confirm to the viewer that this is the The Last of Us universe, I feel that you can make the argument that the abandoned house was a Firefly safe house.
The reason is basically in an analysis of Ellie's vow to "kill every last one of them". It is logical to assume that the at least three dead Survivors in the house are members of this collective 'them'. Who could that be? There are very few large organisations in the post-apocalyptic world of The Last of Us. They could not have been associated with FEDRA - the lack of paramilitary uniforms and heavy weapons is proof of that. Nor could they be a large clan of Hunters. No Hunter group is large enough to provide an epic quest for Ellie to impose upon herself during the game; at most they can be the antagonists for one level.
No, there is only one organisation that is large enough and scattered widely across the former United States of America to support the game and that is the Fireflies.
But why? Why would such a thing be happening?
Ellie's Motivation[]

Six hard years on from Boston
With the focus on Ellie's possible status as the keystone for the cure to the Cordyceps brain infection in the first game, it is easy to forget that this project is something of a side-goal for the Fireflies and certainly not their core objective. As the first game's title sequence reveals, the Fireflies are primarily a political and paramilitary opposition to FEDRA, opposing the effective military rule of what remains of the USA and attempting to restore civilian government by any and all means necessary. Whist a 'nice to have' and a very desirable piece of leverage, the cure isn't automatically critical to that outcome; it just makes the later rebuilding much easier.
Throughout the first game, Ellie expresses her clear hope that the Fireflies might be able to make the world better - that they were the 'good guys' in her own personal universe. Although Joel was cynical he never made much effort to disabuse her of this notion, perhaps recognising how important her illusion that Marlene and Riley's group were the 'White Hats' was to her mental and emotional balance.
The thing is, there are plenty of indications that the Fireflies are nowhere near the noble crusaders for freedom and Constitutional government that Ellie imagines. Beyond their tactics (which endanger random civilians) it is clear that they are not simply interested in freedom - they also want to rule. There are some artefacts recovered in Pittsburgh that back this up. Local are recorded as complaining that the Firefly advisers who had been advising them on their uprising against FEDRA clearly expected to be in charge after the military had been driven out. It is clear that the Fireflies did not just see themselves as 'liberators'. They saw themselves as the natural 'transitional government' to rule over America until legitimate government was restored. History teaches us that 'transitional governments' tend to become permanent institutions until overthrown themselves.
Ellie didn't know this of course. However, maybe she should have guessed. It was no secret that Tommy, a man who Marlene had described as 'good' (a descriptor that she didn't apply to anyone else that we hear), had abandoned the organisation. Why would a man who was clearly idealistic and moral on many levels (to the point where he was willing to take an enormous personal risk for Ellie because Joel had shown him it was the right thing to do) abandon an organisation that was supposedly the 'good guys' - the hope to make something better out of the current mess? I would argue that it is because he saw the organisation's real face. Maybe it was a face that even Marlene was blind to or one that she regarded (like so many other things) as a justifiable and necessary evil.
Setting aside the quest for the Cure, the Fireflies, like FEDRA, must have one overwhelming objective: Like the settlers at Jackson, they realised that no organised state could survive unless it produced resources to support said organisation. Whilst I'm sure that FEDRA controls many farms and factories (which are doubtlessly worked in conditions that border slavery) and that the Fireflies have taken control of those they could, it would be far, far easier for the Fireflies to take control of small, independent resource producers that would not be defended by FEDRA's powerful military. Jackson fits the target profile very, very well.

What would Ellie fight for?
We saw how quickly Ellie bonded with Maria and how much she loved Jackson from the first moment she saw it. This was her Shang-ri-la; her golden city in the clouds that was the refuge from the horrors of the post-Outbreak world. After the loss of hope for a Cure, it can only have grown more important in her heart. Imagine what she would feel for a group that attacked and devastated that home, that sanctuary. The strongest love (and Ellie loved the Fireflies as a light of hope in their own right) so easily turns into the deepest hate.
Or maybe this is about the Cure after all. Maybe the Fireflies have finally hunted down their Keystone, their 'Last Hope' and have threatened to destroy Jackson if she doesn't hand herself over to them. The one outcome that they never expected is that the girl who is the foster daughter of the most fearsome and bloody Hunter in the Central States in the years after the Outbreak might decide that, instead of digging in for a siege or surrendering, she would simply remove the threat. Maybe Tommy and Joel's doubts got through to her over the years or maybe she just can't buy that the 'good guys' would do this. In any case, no matter her view of the Cure and her responsibilities, she will not surrender to an organisation that uses such tactics.
After their return from Salt Lake City, Joel told Ellie that you just keep on finding things to fight for. Maybe Ellie has found her cause for which she will shed blood, Endure and Survive.
She is going to find... she is going to kill... every.... last... one of them for their betrayal of her hopes and illusions.
Possible Romance[]
Amongst the rumours about TLoU, Part II is that there will be a new LGBT character who will be one of the main characters in the game. Given that Bill is hardly a 'new' character and neither is Riley, I'm guessing that there will be a romantic sub-plot for Ellie in the course of the game. You can only wonder whether this will be a redemptive romance or another tragedy that drives Ellie further down the path of hate.
Joel's Motivation[]

The Hunter and his Baby Girl
So, why is Joel standing there, so mild and so gentle? I can't imagine him approving of Ellie walking the dark path he walked after Sarah's death. However, he loves Ellie and, if there is any man on Earth who understands hate and vengeance, it is he. I think that he also respects his Baby Girl enough that he will not presume to tell her what her business is when it comes to fighting for her kin and her home. If she is sure; if this is what she wants to do, then he cannot and will not deny her this quest, this cause and this vendetta.
One thing that his long estrangement with Tommy has taught Joel is that simply saying 'no' and trying to restrain her will force her to break away and do this one her own. There is no better way to ensure that he loses her for good. So, instead, he's going to be there for her; to protect her, advise her and, should it be necessary, remind her of where the line in the sand lies and ask her if she really wishes to cross it.
Of course, there is the Lie. There is good reason to believe that Neil would not have allowed Ellie's discovery of the truth about what happened in Salt Lake City to happen off-screen or, at the very least, the consequence of that truth to be resolved off-screen. So, at some point, there has to be a reckoning between Joel and Ellie about this. Joel will need to have to accept responsibility for this and all the other sins he has committed over the 25 years since Sarah's death and Ellie must decide for herself just what sins her love for her father-figure can forgive.
I suspect that the truth is Joel's final bullet - the last weapon that he is going to keep in reserve to divert Ellie from folly, if that proves necessary.
She is his Baby Girl and he will be with her, through good times and bad. If he can save her from herself, he will; indeed, I think that might be Joel's plot arc in this. The circle will thus be completed: Ellie saved Joel's soul and now he is going to save hers. Whether or not he survives that final and ultimate redemption is something we will have to wait and see.