This game has an interesting concept. I like the simplicity, but wish it had a goal. If there were three to five unique monsters to fight I would like the game a lot more.
Example: Dragon. HP: 10. Atk: 2. Hits on: 15 or higher. Special: Fire breath. If the dragon misses three attacks in a row it deals 3 damage. You may skip your attack to duck for cover. If you duck for cover and the dragon would use fire breath you do not take any damage. If you duck for cover and the dragon attacks you take 2 damage.
You could also add a dodge mechanic. You may roll a D6 before you attack. If you roll a 1 take double damage. If you roll a 6 take no damage. Either way -2 form your next D20 roll (rolls for fighter). This would give the player a choice of what to do rather than being forced to simply attack every turn.
You have good ideas. I would like to see them taken further. :)