Have you never tried Oscuro's Oblivion Overhaul (OOO)? I recently replayed Oblivion+Shivering Isles with this mod and the game is considerably difficult even with the difficulty setting at the 50% mark. I had a lot of fun and probably put in another 300+ hour romp into the game.
And you can put another 100-200h into it if you play Nehrim, the total Conversion, which is absolutely great. I'd recommend Oblivion to everyone, just in order to play that mod.
The Nehrim total conversion even got a higher score in tests than the original game.
