I'm working on a game of my own, and one thing that occurred to me was that a typical housecat is almost a match for a typical 1st-level Commoner. I reasoned that the problem could be solved if we gave all creatures a Hit Point bonus equal to their base Strength score.
For example, a housecat would receive 3 bonus Hit Points (3 Strength), while that 1st-level commoner might receive 11 bonus Hit Points (11 Strength).
This might seem like overkill as far as Hit Point inflation goes, so to make the game grittier, we could keep all Hit Dice in the d4, d6, and d8 range. Furthermore, we could lower the Massive Damage Save to a number equaling the character's base Strength. If a character a 15 Strength, he'd have to roll a saving throw if he suffers 15 damage from a single attack. This is doable if we're looking for something more dangerous....