Anyway, onto the topic. I think that most would agree that finding loot is one of the funnest things to do on Oblivion. However, it was always a "Go into X fort/dungeon/mine and keep looking until you get all the loot".
What I'm looking for is a VERY in depth treasure-hunting quest, or hopefully multiple ones! Now, you wouldn't be forced to do it, obviously, and no one would come up to you and say "Hey, go find this treasure". What my idea is, is that there is a famous, lost treasure that no one has been able to find for over a thousand years, or however long seems fitting. Then, you could be walking around and you'd overhear travelers/civilians talking about said treasure. Seeing as nobody has found it yet, that would leave one to believe that no one has unearthed any clues yet.
This is where it gets fun. This wouldn't be your usual quest where, if you try to do it, you're pretty much guaranteed to do it. This quest should make the treasure SO hard to find, that most gamers wouldn't be able to find it, or at least, it would take them a very long time to find it. Only the hardcoe fans would put in the work necessary to unearth all the clues, put them all together, and then figure it all out. Also, the main thing I'm thinking of is that the game would NOT give you any help in figuring it out. The clues could be correlated to Tamrielic lore, where as they are basically very vague clues that don't make much sense. This is where the player comes in! You'd have to actually try to figure out what each clue means, and the game would not give you any further help on the matter. Let's say you found around four clues; Usually, at this point, the game is telling you where to go. Like in Oblivion: "X NPC told me this, and Y NPC told me this, so I should go here". No, I don't want the game to give you any help at all. That way, you would really feel a sense of great accomplishment when/if you figure everything out and find the treasure.
To put it easier, I want a treasure quest, or quests, that are similar to the movie "National Treasure". "The secret lies with Charlette" is a perfect example of a very vague clue that correlates to a certain history/lore. Only the devoted few would be able to figure it out, and it would be a heck of a lot of fun. Plus, finding the most famous lost treasure in the history of Nirn would gain you considerable fame and wealth, and your name would be remembered for something other than the main quest!
Sorry for the wall of text, but I've been dying to see something like this in ANY game, a mission where you, the player, have to do ALL the work, and there is a good possibility that you may never figure it out (without the help of the internet, of course)! It would put some much needed thrill into any game, so why not TES V?!