I might just be overthinking it, but I feel like there could be something special about the giant Darkside in the KH4 trailer.
Ava says that the darkness was given form once they (presumably the Foretellers) arrived in Quadratum. I believe that the Darkness is following them specifically. We know that it would follow the MoM relentlessly no matter where he went, and he was hoping that he could finally escape from it if he went outside of reality.
The Darkside and the other Heartless appear to be more human-like in Quadratum. It makes me wonder if this Darkside is so gigantic because its form was based on someone in specific - someone very strong - like the MoM. If that's true, its appearance might be a hint towards what the MoM looks like.
You may notice that it doesn't have the "tentacles" that usually cover a Darkside's head and face. Instead, it has some "hair" that falls over its face and spiky hair on top. (I'm trying so hard not to say that it looks like Jack's hair. I truly might be hallucinating. I see Jack in everything.) Even if its features don't end up being unique, I think its sheer size will be; I doubt that every Darkside we encounter in KH4 will be the size of Godzilla like this one.
It isn't the first notably unusual Darkside we've seen...
"It looks different from other Darksides Sora has battled with, but the cause remains a mystery."
Even the journal points out its strangeness. This mystery has plagued me for a long time. I feel like most people forgot about this or just shrugged and never thought it was that important to begin with.
Why did this Darkside from the Final World look so different? It's either made of light and darkness -- or water, since it seems to be see-through.
Anyway, based on how difficult the battle against this Darkside seems to be, I don't think Sora will be able to defeat it by himself. Is that when Xehanort steps in? Or will the MoM himself take it down?
Edit: I've been reminded that Donald and Goofy exist. The two lights in the trailer could be D&G to the rescue. lol (But will they be enough?)