Frost Craige Spire is more interesting with zero upgrades than the theives den is maxed out, (...when my Bosmer thief character got there, he's like "Oh, umm this couldn't have been intended for me. It must be a mistake. Some wizard is going to turn up a blow me a new hole in the face. ... but while i'm here... IC: am totally freaked out about the place, and have only snuck backing once or twice to steal potion ingredients.)
I had Kinda hoped the theives den would get a bit better with each Upgrade, but no, basically they each just add an NPC (except maytbe one?) and a couple of light sources/items related to that NPC,
a bit of a ripoff for 1000 gold (esp since the NPCs all overlap massivly).
The secret entrance to the Castles neat (and I have half a mind to start using some of that space), but even so...
In MW there were some great improvements for the Great House strongholds (which are the closest things to the house DCPs in my mind), but I can't find any improvements the the Theives Den.
Surely there must be some mods that make there theives den sweet.
Things like making it more Acrobat accessable (its ... Ok, but still pretty annyoung thrying to jump from water lvl to the ships deck)
And ... just making it interesting