Instead of the No Backwards running mod above, try to find Duke Patrick's Backwards running risk. You can still run backwards, but if you run into anything (including dead bodies) you will fall. Supposedly, it is possible to run backwards in a circle continuously, but I have not been able to do that yet.(This is not a cheatery tactic, it was designed that way, apparently such a tactic is actually used by SCA fighters.)
From what I read, most of them actually increase the player's capabilities and reduce challenge.. for example, the first mod listed lets you exceed your encumbrance limit for a significant combat penalty. The default game makes it impossible to move at all if you exceed encumbrance!
I use ABO's Realistic Fatigue. It could be described in a similar way, but trust me, it makes the game FAR more difficult. In Vanilla Oblivion, you can be one encumbrance unit below your maximum, and have no penalty. But With RF, you generally want to be at 25% or less of your maximum encumbrance, (which is increased) as all actions take far more fatigue when you are more heavily encumbered. When heavily encumbered, you will not be able to run very far before running out of fatigue. When low on fatigue, you take a chance of tripping or stumbling. When you run out, you will fall. Combat is already dependent on fatigue without combat mods. Just imagine being held up by a lowly highway man back into town after you foolishly tried to load yourself up with loot after sacking a dungeon.
Consider basing your game on OOO. It will significantly ramp up the difficulty, but in a realistic and very cool way.
If you can get a hold of Duke Patrick's, combat mods, do that as well. These will make combat much more interesting, and fast paced.
As you start adding more mods, you will want to get rid of the Ironman mod. If you run into a glitch that kills you, you know what happens. And besides that, getting used to Realistic Fatigue and OOO will have you end up dead many times. Especially if Duke's combat mods are in the mix.