Perhaps a mod like this has already been created - but if it has, I haven't found it.

I got to thinking about ways to continue improving the realism of Oblivion. One such way would be to have treasures, enemies, etc. be slowly restocked over time after a dungeon is cleared, rather than the current method of "wait 3 days and everything's reset".
For example, perhaps 3 days after you first cleared a Goblin lair, only a couple of Goblins, a few rats, and a couple of filled treasure chests are available. After 7 days, a few more show up; after a month, the dungeon is back to half capacity. Etc.

In addition to this, perhaps a few dungeons would change "enemy types" after you've cleared them. For example, perhaps a fort full of Undead would become a Bandit Lair after you clear it out. Or vice versa.

Finally, perhaps a few random adventurers (not the type that randomly spawn in a dungeon, but that continue to be processed until they're killed) could compete with you to find the best treasure. If they raid a dungeon before you, then too bad - there's not going to be much left for you to find at that time. They may even take the fact you're encroaching on their ruin personally, and decide to attack you with some of the awesome equipment they just found.

Just a thought.