I was afraid I'd hate it after spending so much time on my own canon. Not because it was bad, but because I was so invested in my own work. Fortunately I can say I only really didn't like one thing: The Tree of Harmony as a macguffin. If it had been located in the celestial plane where Twilight ascended and/or been explicitly a metaphor, I would have liked it better (a thing can be both real and a metaphor, after all...), but as it is it feels...kinda weak.
I absolutely loved the dual day night. And I wish I'd thought of it. Celestia and Luna seem to be not only the controllers of their respective bodies but also incarnating the actual metaphysical spheres of day-ness and night-ness. Honestly I feel that, Canterlot Wedding aside (grrrrr!), Tia and Luna as Gods is fully supported by canon. Obviously they can't/won't make it as explicit as I have but I don't expect that.
I really liked the Nightmare Moon bit. She broke the balance and that turned her into Nightmare Moon. It wasn't some external thing, it was the result of her own actions and somewhat a backlash/followthrough of violating that separation. Eclipses apparently don't happen in Equestria unless Luna's doing something she really shouldn't. There was definitely solar magic bleeding into the Lunar during the transformation sequence. Which makes me wonder how exactly Celestia manipulated the moon if the spheres are that separate.
Discord was Discord. The fact that it was his magic that grabbed Tia and Luna makes it palatable since he's already defined as having no upper limits outside of the EoH. I liked that they explicitly yanked his friendship-chain and weren't much impressed with his attempts at manipulation.
I feel Twilight could have been a bit more competent. If nothing else she needs to learn how to fight properly and react properly to threats! And was that Sombra type magic that she used on the history potion? What the heck was that about?
And finally, whatsinthebox?
So, it seems clear that in S4 there are two thematic arcs they need to accomplish. Balancing princessing and her friends (explicitly stated) and finding her own sphere of influence/expertise as a princess (not so explicitly stated). I'm fairly sure the box is aimed at the latter. Six keys means yeah, one key per mane character of course, which sort of implies an EoH replacement. Twilight's cutie mark being on the trunk is not, I think, necessarily destiny - after all, Shining Armor has it on a shield as his cutie mark - but it does suggest that Twilight does have a specific connection with whatever the Tree of Harmony represents. Probably one of many unicorns to have it, but not all become alicorns.
Also, I should give props to Kaorin. In Triptych I talk about Twilight's friends being her Guard. And they precisely are in the premiere. That was all Kaorin's idea but holy crap is it rich to grab and run with now.