If you wanted to mediate the differences in the classes a bit, you could change the HD progression to something more like this:
Barbarian, Fighter, Paladin, Ranger: +1 HD per 2 levels
Cleric, Druid: +1 HD per 3 levels
Bard, Rogue, Monk: +1 HD per 4 levels
Sorcerer, Wizard: +1 HD per 5 levels
This would make a Human 20th level fighter a 1d10 + 10d10 character. The first getting max HP (10), assuming average rolls (1d10 average = 5.5 per die) means 65 HP before CON is taken into account.
This would make a Human 20th level Cleric a 1d10 + 6d10 character. Again, the first getting max HP (10), assuming average rolls means 43 HP before CON is taken into account.
This would make a Human 20th level Bard a 1d10 + 5d10 character. This results in an average of 37.5 HP before CON is taken into account.
This would make a Human 20th level Wizard a 1d10 + 4d10 character. This results in an average of 32 HP before CON is taken into account.
On average, Half-Orcs would have +1 HP per HD over humans (+2 on the first one). The D8 races would lose 1 HP per HD on average (-2 on the first one). The d6 races would lose 2 HP per HD (-4 on the first one).