Today I made a big effort at opening, exploring and reading through the 1.8 patched TESV.exe executable (the main file of Skyrim), attempting to find as much of the pre-Dragonborn data as I could in the files. While this may bore some, it may be interesting to others. Keep in mind that some of the content may have existed for a longer time, having remained unused. Most of it however is Dragonborn content, prepared in the 1.8 Skyrim patch.
This is what I could find so far:

Holidays

• New Life Festival.
  
• South Wind's Prayer.
  
• First Planting.
  
• Jester's Day.
  
• Second Planting.
  
• Mid Year Celebration.
  
• Merchant's Festival.
  
• Sun's Rest.
  
• Harvest's End.
  
• Tales and Tallows.
  
• Witches' Festival.
  
• Emperor's Birthday.
  
• Warrior's Festival.
  
• North Wind's Prayer.
  
• Old Life Festival.

Holidays! They are mentioned right after the day and month names in the file. Maybe when the day is a holiday, special events happen! Discount from Dunmer merchants on a Dunmer holiday, gifts from Imperial NPC's around Emperor's Birthday? Sheogorath appearing on Jester's Day? Lets hope it is something nice!

General stats menu

• Dragonborn Quests Completed.
  
• Solstheim Locations Discovered.
  
• Stalhrim Items Crafted.

Stalhrim Items Crafted is an interesting stat. Since it is the only item material that is counted, it may well mean that it is very powerful or difficult to make. Do we finally get a set of weapons and armour which we really have to earn and work for?

New Weapon categories

• Stalhrim.
  
• Nordic.

Nordic weapons are interesting, they are connected to the prefix DLC2 so they definitely are not the nord patterned steel weapons we already have in Skyrim. Stalhrim weapons we saw in the screenshots already.

New Armor categories

• Heavy Stalhrim.
  
• Heavy Nordic.
  
• Heavy Chintin.
  
• Heavy Bonemold.
  
• Light Stalhrim.
  
• Light Nordic.
  
• Light Chintin.
  
• Light Bonemold.

Interesting is that we get both light and heavy versions for armours. Finally no anger over not getting that particular armour that fits your class.

Dragon Mount related

• Player can mount dragon here list
  
• Dragon mount no land list.
  
• Flying mount - allowed spells.
  
• Flying mount - disallowed spells.
  
• Flying mount - fly fast worldspaces.
  
• Help - flying mount

Dragon Mount messages

• "Your dragon cannot land at this time."
  
• "Your dragon is searching for a place to land."
  
• "You cannot cast that type of spell while riding a dragon."
  
• "Cancelling your request to land."
  
• "Your dragon cannot fast travel right now."

Item related

• Cuirass.
  
• Verlet Cape.

Verlet is a physics system to make things move like they do in realtime. A verlet cape would be a cape that actually moves depending on both the direction of the wind and on how you run and move.

New Location related

• DLC2SolstheimWorld.

So far no mention of Oblivion or such. Also nothing on the red mountain, after seeing that screenshot. Who knows, new stuff may be added with a next patch before release, or it is found in the DLC2 master files on release.

PLEASE credit or refer to me, Saratje, if you are reposting this somewhere. Thanks!

Some of this content is bound to be new.

Most of that stuff is old, but the holidays part is new

A lot of it was new to me, at least. I hope even half of this stuff is expanded and implemented into the final version of the DLC.