So, about Serana

FabresFour

Many people say that Serana was already asleep during the Second Era due to a phrase from her involving the Dwemer. However, she was born and lived during a period without an Empire, which may imply that she was born before the foundation of the First Empire or during the ESO period.

However, according to Matt Grandstaff, Senior Community Manager at Bethesda Softworks: "The intention was that Serana went to sleep in the late second era, between the Reman and Septim empires. Her initial dialogue is just her surprise that there’s an Empire in Cyrodiil, as there hadn't been when she went to sleep".

Even if Zenimax said that it will not involve the Volkihar in the next DLC, will we see Serana in any way?

VaranisArano

We've already had a thread about this: https://forums.elderscrollsonline.com/en/discussion/504358/next-chapter-involves-volkihar-vampire-clans-formation/p1

Including some other sources from the developers. They aren't as consistent or cut and dried as your source makes it sound.

Here's an ESO AUA citing a 1st Era date for imprisonment, specifically in response to a question asking if Serana and family would show up in ESO: https://www.elderscrollsonline.com/en-us/news/post/699

Here's another ESO Writer's AMA from Sept 2014 saying "Serana is, IIRC, already in her sleep-tube by the time ESO kicks off." In response to a question about whether Serana will be in ESO. https://www.imperial-library.info/content/zenimax-online-writers-reddit-ama

None of that is to say that the Devs/Writers can't backtrack or contradict what they said earlier, but its not quite as cut and dried as the first quote makes it sound.

When you add in the Skyrim: Dawnguard dialogue from Durnehviir and the timing of Vyrthur's prophecy that kicks the whole thing off, it gets really hard to argue for a 2nd era date for Serana going to sleep. Certainly the Skyrim writers did not have ESO: Elsweyr in mind when they had Durnehviir date his entrance to the Soul Cairn and the bargain to guard Valerica as a time when "The dovah roamed the skies, vying for their small slices of territory that resulted in immense and ultimately fatal battles."