I don't see why there can't be fixed level dungeons right from the start.
How about multi-tier-leveled dungeons?
Let me explain.
Say you're in bleek falls barrow (since thats the only place we know of.). Perhaps as you venture deeper into the cave, (Bleek falls barrow underpass, bleek falls barrow underground basin...etc..) the enemies become higher levels.
So maybe i'm only strong enough to get through the first floor of a dungeon and I have to come back to it to complete the later ones? You still got in the cave and did some killing...you weren't necessarily locked out.
That makes every single dungeon accessible and also gives you a reason to come back or risk fighting for your life and wasting all your resources to push through. None of this dungeon locking nonsense -- I could easily exploit that if I wanted to.