Well, all of them and there are a few other I would like to see, among which :
http://www.uesp.net/wiki/Bloodmoon:Castle_Karstaag of Frost Giants
Ship wrecks
Border walls
Warden bridges
Sewerage Systems
Mines
Barrows and Tumuli
Forgotten secret passages that lead to a nobleman's castle
Towers
Haunted houses and manors
Witches convents whose inhabittants have left and have been replaced...
Dunmer ancestral tombs near the Eastern border
Dunmer strongholds near the Eastern border (like the ones with the Propylon chambers)
http://www.uesp.net/wiki/Lore:36_Lessons_of_Vivec,_Sermon_36, with remnants of the clockwork dreughs, once the minions of Sotha Sil, off shore, under deep water
Vampire icy manors owned by the http://www.uesp.net/wiki/Lore:Immortal_Blood, under icy lakes
Ruins that belonged to extinct human races mentioned in http://www.uesp.net/wiki/Lore:The_Adabal-a, among which the Nede and the Men-of-'kreath. I don't know how many of these races come from Skyrim.
Riekling settlements
Grahl settlements
Perhaps some open air "dungeons" such as a cromlech with Centaurs or Minotaurs or Udyrfryktes, or such as a Spriggan settlement in (http://www.uesp.net/wiki/Lore:Special_Flora_of_Tamriel?) trees.
Also, some particular places such as
http://www.uesp.net/wiki/Lore:Pocket_Guide_to_the_Empire,_1st_Edition/Skyrim with its huge irregular blocks of porphyry
http://www.uesp.net/wiki/Lore:Saarthal
http://www.uesp.net/wiki/Arena:Labyrinthian
http://www.uesp.net/wiki/Shadowkey:Fearfrost_Caverns
http://www.uesp.net/wiki/Shadowkey:Lakvan%27s_Stronghold
http://www.uesp.net/wiki/Shadowkey:Crypt_of_Hearts_I
I hope I haven't been too enthusiastic...
A very nice list you put together there, I appreciate the enthusiasm
Ahhh Labyrinthian, I have never played Arena myself but from what I hear it was a really long and mystical dungeon and I would love to see it in Skyrim although as long as it isn't just copy-pasted with an HD makeover, I once read a thread about the Labyrinthian being in Skyrim and one of the posts said something like; "Reimagine it, don't port it" so yeah, pretty much that.