Very early in the game, like pre-version 1.0, elements were added to allow for future content. A lot of those elements were used. A lot were not. For future reference, here are some of those elements that just didn't happen.
1) The tower in Southern Bluemist River, the one you mention.
2) The second map of the Lannager mine. Those rocks will just keep that staircase buried.
3) Missing Doug. Not only did he end up missing for years of development, the NPC that mentions him had a dialogue changed to no mention him anymore. That means that Doug is less real than the Invisible Swamp Donkey (Which has atleast a partial in-game reference now!).
4). Crafting! Yep, there's a few NPC's that still will trigger a reference to crafting coming in a future update. Don't count on it.
5) More magical stuff for Warriors. The warrior skill Magical Training was planned to allow use of a few magical weapons and items for the warrior. Now, it's a rather useless skill. Sure you can learn the skill to allow the use of a few scrolls, but not really worth it.
6) The missing bolt element dungeon in Tower of Tremadan.
7) Using the non-existent third elemental gem to power up a weapon (see item 6 above). The game only has two such gems, one to open up the tower, one to open up the lab area.
8) A few doors in every capital. I'll lump them all together as one item. There are a few places, buildings etc, that just don't have anything even though it seems like they would.
9) Areas on the world map that don't exist. Like Item 8, lumping multiple items under one list. You can't get to Mercia's port towns, or the Orc Kingdom. Same with those pesky pirates.
10) The final story arc involving Syllara the witch. In-game references and quest options set up a final showdown between Syllara and The Thee. However, all you really can do is cause a respectable town guard to go missing (hey, stun immunity is so worth it.... right?)
I'm leaving the haunted cabin in Wyvern Mountains off this list because I'm not sure if was supposed to be used in a quest that never happened, or if it's just an easter egg as a reference to the Evil Dead ???