Theorizing, Season of Dawn: Process of Elimination
I want to quickly point out that they have don't have the same threat featured two times in a row. (Or at least not linked in some way).
D1 Order: Vex, Hive, Fallen, Taken, Fallen
D2 Order: Cabal, Vex, Hive, Scorn, Fallen, ? , Taken, Hive, Vex.
So that eliminates Vex from the equation. Hive can also be eliminated unless we are talking about the Frozen hive and Rasputin (more on that later).
I think it is fair to say we are not talking about the scorn (lack of leader, doesn't fit with the term dawn, etc.)
Now let's talk about possibilities:
I don't know much of the lore, besides what I've heard others talk about here.
[b]The House of Light[/b], Mithrax, etc. I don't think is specifically likely, due to one, where would this be, and two, that seems like such a big change. It would only fit as a big update I think.
The next two focus on the phrase 'a new dawn', and similar phrases. [b]First, Rasputin:[/b] I don't really know what we would be working with him about, maybe it's Eir, as that worm god is probably not happy we kicked Xol's butt. But I don't know. The teaser from bungie is still an interesting one though, and that's why I brought it up.
[b]Second,[/b] we look at the Cabal. It just seems like they need something. Cabal I feel are sort of odd. They've appeared once, almost won, but that's it. We have Calus still sitting there, and Caiatl could be bringing something (daughter of Calus, Cabal emperor).
[b]Dreaming City:[/b] My thought was, well we wake up at dawn, at when we wake up, we stop dreaming. Then maybe be clean this sh*t up? I heard someone mention Xivu Arath has taken stealing stuff there, but I don't know if that's true. But we still have Savathun, and it would be interesting.
My last idea (and my weirdest one) comes from the phrase 'it dawned on me'. this takes us looking for answers about the pyramid. I don't know where, or how, or why, or anything that could answer this, but what do you think?
P.S - Sorry, I don't know much about Season of the Drifter, I didn't play it, I thought of putting the nine in there, but didn't either. Yes, I separated Shadowkeep and Season of the Undying.
P.P.S - If the lore counters any of these ideas, well, I haven't read it. And if all of these ideas, then I just wasted your time and my time, and sorry about that.