According to Bethesda forum member Mardoxx, who did a little bit of digging in Skyrim's 1.8 patch source code, Bethesda may have another Skyrim DLC, dubbed "Dragonborn" in the works.
His post on the official Bethesda forums points out the following evidence:
"data\interface\Translate_english.txt
$Crafting_$DLC2ArmorBonemold BONEMOLD $Crafting_$DLC2ArmorChitin CHITIN $Crafting_$DLC2ArmorNordic NORDIC $Crafting_$DLC2ArmorStalhrim STALHRIM $Dragonborn Quests Completed Dragonborn Quests Completed $DRAGONBORN_ESMName Dragonborn $DOWNLOADABLE CONTENT_PS3 DOWNLOADABLE CONTENT
update.bsa\interface\exported\hudmenu.gfx & quest_journal.gfxHudmenu compass markers/quest journal quest header:http://i.imgur.com/mWGUj.png Clockwise from top, Telvanni Tower, To Solstheim, Raven Rock, Castle Karstaag, Standing Stones, To Skyrim, Miraak Temple. Bottom is quest header for Dragonborn quests
update.bsa\interface\startmenu.swf\startmenu.as:
MainList.entryList.push({text: "$DOWNLOADABLE CONTENT" + (iPlatform == 3 ? "_PS3" : ""), index: StartMenu.DLC_INDEX, disabled: false});
Update.bsa\meshes\animationsetdatasinglefile.txt
DragonMountedDualMagic.txt DragonMountedDualStaff.txt DragonMountedMLhSolo.txt DragonMountedMLhStaff.txt DragonMountedMRhSolo.txt DragonMountedMRhSolo_ShieldOrTorch.txt DragonMountedMRhStaff.txt
There are other things like new creatures and dwemer objects, but they're not that interesting."
Though Mardoxx's forum topic was locked, others have been started to carry on the speculation. So far, it's been uncovered that Dragonborn may feature spears, new armor types, and the one thing we've all been yearning for out of Skyrim: dragon riding.
Solstheim was the setting for Bloodmoon, Morrowind's werewolf expansion. However, considering there are already werewolves with the Companions faction in Skyrim, it's highly unlikely that the conflict that'll be present in Solstheim will revolve around werewolves. But who knows? Maybe this will be the werewolf DLC that parallels the vampires in Dawnguard.
