The naming conventions of the new Cyrodiil campaigns. They have to be whats playing an important part of the chapter. We know Ravenwatch is going to return. But another name that was most interesting was the Gray Host looked it up and saw the lore on them.
https://en.uesp.net/wiki/Lore:Bangkorai,_Shield_of_High_Rock
Mentions it as "Then, in 1029, when the legions of Empress Hestra deposed King Styriche, the Vampire of Verkarth, he fled west at the head of his dreaded Gray Host, burning and killing as they came. But when his army of bat-men and wolves reached the Bangkorai Garrison, they broke like a wave on a rock. Hestra's legions caught and killed the survivors, and the Empress was so impressed that she honored High Rock with admission into the First Empire.

In anchors sometimes King Styriche, A werewolf and a bat which I assume werebat are summoned sometimes by Molag Bal to deal with us at the anchors. But that does not mean we are not going to see the reformation of the Gray Host. Given the name, and we do know that there might be a vampire/werewolf city or alliance. I do believe we are going to see a reformation of the Gray Host or the breaking imprisonment of many of its most powerful members. Just because it makes sense.

I suspect haven't played it still downloading pts. That the Unhallowed Grave Content will be what will introduce the return of the Gray Host. The reason being its location in Hammerfell. The Grayhost was based in the city of Verkarth which happens to be in central Hammerfell possibly in the Fallen wastes region which is located in Southern Bangkorai.
https://en.uesp.net/wiki/Lore:Fallen_Wastes was mentioned in the Bangkorai Shield of high rock.
https://en.uesp.net/wiki/Lore:Verkarth_(Hammerfell) is also mentioned as well. I suspect they might have magically imprisoned many of the members of the Gray Host. In the region where the new dungeon dlc is going to be but not necessarily in Verkarth.
https://elderscrolls.fandom.com/wiki/Verkarth
Also goes more into depth into the history.

Now Ravenwatch I suspect if they are going to be involved can be explained by their connection to King Emeric he might have asked them to personally look into it. Whatever happens in Unhallowed Grave will possibly be what goes into the new chapter but would also be a big threat to High Rock and not just Skyrim. The reason why I think we are going to see the return of the Grayhost is because it ties into the Reachmen vibes that seems to be hinted at in the Mysterious Parchment. That and the name of Verkarth really does relate somewhat to the name Markarth which is a city under control of the Reachmen in the time period we are in.

So The Icereach witch clan will be involved Grayhost will be involved House Ravenwatch will be involved and Blackreach will also be involved. I suspect that the Reach and the Despot of Markarth will also be involved in it. Given they hinted that might be the location of Forth Quarter. So I do think that is what the storyline is going to be about. As for Harrowstorms that is still a mystery. Not sure how those will play into it all. But I'm sure going by the name of the Cyrodiil Campaigns we are going to see the return of the Gray Host. So we will be facing not only vampires in the new chapter but also a joint vampire/werewolf alliance and maybe witchs and reachmen also joining in. If I'm correct about this.