This post is not to predict what DawnGuard is (as the title elicits) but to help the many accusations over the expansion's contexts be more accurate. Now, the way I hope one will facilitate more accurate guesses is by taking more consideration into Skyrim's literature, which will, no question, open up more theories for debate but will also abate other opinioins. My goal is to help the community narrow conjecture on the basis of proof that Skyrim provides, replete in texts.

While not having read all the books of Skyrim, I have read over half in my collection which consists of more dark and theocratic ideas. Of the books I have read, many that resuscitate the history of Tamriel have come to offer some of the best hints at what DawnGuard may hold. The titles I will share with you include:
Ancestors and the Dunmer by unkown,
Daughter of the Niben by Sathyr Longleat,
Father of the Niben by Topal Pilot,
Fire and Darkness by Ynir Gorming,
Glories and Laments by Alexandre Hetrard,
Life of Uriel Septim VII by Rufus Hayn,
Rislav The Righteous by Sinjin,
Sovngarde: A Reexamination by Bereditte Jastal,
The Adabal-a by unkown (while this book offers insight into the Ayleids--future Altmer--and St. Alessia, it is very complex and requires prerequisites of other readings to understand certain terminologies),
The Amulet of Kings by Wenengrus Monhona,
The Arcturian Heresy by The Underking Ysmir Kingmaker,
The Book of the Dragonborn by Emelene Madrine,
(continued)
Words and Philosophy by unknown,
The Lunar Lorkhan by Fal Droon,
Skyrim's Rule by Abdul-Mujib Abadneth,
The Oblivion Crisis by Praxis Sacorum,
(one of the most important) The Monomyth by Unknown,
The Rise and Fall of the Blades by Anonymous,
Dwemer History and Culture by Hasphat Antabolis,
(another big one) The Alduin/Akatosh Dichotomy by Alexandre Simon,
and The Great War by Legat Justianus Quintius

The books above as I had said are not, and by no means, all of the texts of Skyrim that will offer insight to DawnGuard. These are just some of the many I have come across which will aid in determining what we can propose will be in DawnGuard by reading of regional tensions, understanding races' community and religious standards, and being open to the history of Tamriel. I do hope that my theory towards better understanding DawnGaurd through such readings opens up more intellectual debate in the community. If anyone comes across other readings of potential guidance, do post below and I will do my best to take a reading, determine its value, and add it to this post. Also if anyone finds any of these readings to not be beneficial, please do state why, and if valid and I can offer no argument against it, the book may be repealed so we can better evaluate DawnGuard.

I have gone through my collection of 61 books. I do own many others but I had discarded them and placed them safely in a trunk. I will re-evaluate them to see if they are of any use.

This reminds me of when Oblivion was first announced and I scrounged up all the books on the subject in Morrowind hoping to gain some insight.

Best answer to all speculation yet. Will follow this.

I placed down some more