I guess I'll also throw out if people consider incorporating the "Masters of the Wild" feats as a better solution.
- Dwarf's Toughness: Prereq Fort base +5; +6 hp.
- Giant's Toughness: Prereq Fort base +8; +9 hp.
- Dragon's Toughness: Prereq Fort base +11; +12 hp.
The problem with those feats is that they are inferior to Imp. Toughness. All except the second are equal to Imp. toughness at the earliest level they could be taken and worse every level afterwards. The second is just plain worse.
How about it granting you extra hp = your HD value? For example, fighters get another d10, wizards another d4 (not sure if con mod should factor in, or if players should roll or simply use a fixed value, or how this would interact with multiclassed characters though). I like the idea of the extra hp varying according to the class you are playing.
Me too, but I don't want more HD rolls to keep track of (though, I currently have fixed HD rules, so it'd be less of a problem). I think I have a good idea to re-write Toughness and Imp. Toughness, the only downside being that you can't take Toughness repeatedly anymore, which will alter some monsters (like the triceratops). Also, this toughness is MUCH better, so you'd have to alter Dwarven Defender pre-reqs and others, as frankthedm noted.
Toughness [General]
Benefit: You gain an amount of bonus hit points equal to twice your BAB (minimum of +1 hp), which scales as you gain levels.
Improved Toughness [General]
Benefit: For the purposes of the Toughness feat, your BAB is considered equal to your HD. In other words, you would gain +2 hp per HD. This benefit overrides the benefit of the Toughness feat.
So, this feat benefits warrior types much more than others as the levels climb, but mages can just spend a second feat to catch up. The option of the second feat leaves medium BAB classes in a weird gap where it's just not worth it to spend a second feat, but I think that's ok. IME, most who take the Toughness feats are either meat shields looking for as much hp as possible or d4 HD arcanists, trying to offset their low HD.
And, since too often a guy with giant wads of hp as his main defense just ends up as a drain on the party's healing, I present an idea for a new feat. It could use a better name.
Resilient [General]
A combination of physique and will power allows you to heal from wounds and injury much faster than a normal person could.
Prerequisites: Toughness, base Fortitude +3
Benefit: Whenever you are healed of hp damage, you heal an additional amount equal to your BAB (or HD, if you have Improved Toughness), to a limit of the amount of original healing. A Cure Minor Wounds spell would heal you 2 hp, a Cure Light Wounds from a level 5 cleric could heal you as much as 26 hp if you had +13 or higher BAB. This also affects your natural healing rate, so a character with full BAB would effectively heal twice the normal amount from a night of rest. This benefit applies to both lethal and nonlethal damage that you heal.