I love making speculations on update features. I see it like a game. Feng likes to tease us with gifs and I like to pick out all the possible features and how they will work.
Character Customization: Not confirmed for sure but it looks that way, green mohawk and the ability to have the jacket on or off. It seems like you will be able to choose what head and what body you want for your custom character. My idea was that it will be linked to your account and you can choose it when you select a character. I am just thinking if he will add rico, hanji, connie, annie, hatsune miku, reiner, bertolt to the character select menu as he has stated that there will be no new special moves.
Custom Stats: So there was a comment that Feng posted on his blog post that translated to "stat customization" so it is believed that you will be able to choose how many points go into what stat. This will be interesting now seeing how people build their characters to their playstyle and what they are willing to sacrifice; blade and gas for speed and accel. I can't wait to try it out. Again I think that this could be linked to your custom character that may or may not be linked with your account. Fingers crossed.
New Animations: We all saw em, how pretty! I really like the new animations, it gives an epic feel to landing and makes maneuvering with the characters look more alive and real. The gliding backwards is cool cause you can see that the head moves to looks behind almost, pretty cool to see it's not stiff anymore. The landing looks really good I can't wait to see it in game for myself :p
New Characters: Touched on in custom characters but there are a number of new characters shown in the gifs:
RicoHanjiConnieAnnieHatsune MikuReinerBertoltAre all the ones I recognized. It's cool to see that the hair physics are looking nicer, not stretching and silly looking. At least that's what it looked like, maybe he wasn't going fast enough. Rico also has a garrison cape which looks pretty snazzy too. I am just not sure about whether all the new characters will be added to the roster in game. Maybe Feng was clever enough to get rid of the roster and just have you chose a character to be linked with your account, not to sure about this theory though as guests would not work, maybe they would have a normal basic character with basic stats and a base model (Mikasa?). This would be good as choosing a character everytime would be a pain. And with custom stats linking with that, you wouldn't need to change character mid game, it might also mean camera style could be saved to your account.
Extra Findings:
the previously released picture of the mysterious coat. My first suspicion was that Shadis would be added as he is know to wear a long coat like the one in the picture.
The reason why they don't use long coats if you didn't know is because it would get in the way of the 3DMG. Maybe now you can use a long coat? It's not in the gif though so don't get your hopes up.
If you have any other theories I would be glad to hear them. I wish some of the things noted above are added, it would be nice but we can only hope for the best.