This might be an interesting feat to add, with some modifications:
Improved Natural Attack ( Monster Manual III, p. 206)
The natural attacks of a creature with this feat are more dangerous than its size and type would otherwise dictate.
Prerequisite
base attack bonus +4
Benefit
The damage for one of the creature's natural attack forms increases by one step, as if the creature's size had increased by one category.
Anyone who's not a monk could perhaps have their unarmed strikes improved to a 1d4? 1d6?
If the character IS a monk, however? Why not make it so that their unarmed advancement is one grade higher then their level should give?
Like this:
Original Damage dice ==> New damage dice
(Damage taken from level 5, which is the earliest it can be taken on Monk)
Large/Medium
1d8 ==> 1d10
1d10 ==> 1d12
1d12 ==> 1d20
1d20 ==> 3d8
Small
1d6 ==> 1d8
1d8 ==> 1d10
1d10 ==> 2d6
2d6 ==> 1d20
This way, Monks who invest in this feat have their own, similar version of Greater Weapon Specialization (Unarmed)....And it would perhaps make people willing to break away from pure Monk and try out prestige class combos?
Also, perhaps even have some more smaller subraces being monks?
Anyway, just a thought, not sure if it's feasible, even.