I have the answer! The answer lies not within the hp, but the effects!
Okay, below are my revised suggestions for the Toughness line of feats. To utilize these feats, however, another House Rule must be applied.
With regards to "Dying", having only a 10 hp window throughout your career is not only unrealistic, it's useless at higher levels. I realize that the Epic Level Handbook touches on this, but their suggestion does not deal with the problem well enough, so I have come up with another House Rule:
Characters don't actually "die" until they reach a negative amount of hp equal to their Con + Character Level.
With that in mind, let's revisit Toughness; the following feats stack with each other, but each may only be taken once:
Toughness: gain 5 hp, "death's door" extended 10 hp
Dwarf's Toughness [Base Fort save +5 or higher, Toughness feat]: gain 10 hp, "death's door" extended 10 hp
Giant's Toughness [Base Fort save +8 or higher, Toughness feat, Dwarf's Toughness feat]: gain 20 hp, "death's door" extended 10 hp
Dragon's Toughness [Base Fort save +11 or higher, Toughness feat, Dwarf's Toughness feat, Giant's Toughness feat]: gain 40 hp, "death's door" extended 10 hp
Epic Toughness [Epic Feat; Toughness feat, Dwarf's Toughness feat, Giant's Toughness feat, Dragon's Toughness feat]: gain 80 hp, "death's door" extended 10 hp
And now for a new feat to be the climax of the chain:
Improved Epic Toughness [Epic Feat; Toughness feat, Dwarf's Toughness feat, Giant's Toughness feat, Dragon's Toughness feat, Epic Toughness feat]: gain 100 hp, "death's door" doubled
On top of that, while I'm on the subject of life-extending feats, here a new one to ponder:
Immortality [Epic Feat; Con 20+, Base Fort save +40 or higher, Heal 30 ranks, Toughness feat, Dwarf's Toughness feat, Giant's Toughness feat, Dragon's Toughness feat, Epic Toughness feat, Improved Epic Toughness feat, Great Fortitude feat, Epic Fortitude feat, Iron Will feat, Epic Will feat, Endurance feat, Epic Endurance feat, Remain Conscious feat, Extended Life Span feat, Great Constitution feat, Fast Healing feat, Perfect Health feat, Skill Focus (heal), Greater Skill Focus (heal), Epic Skill Focus (heal)]: live forever, "death's door" extended 100 hp, you no longer suffer ability penalties for aging and cannot be magically aged, although any penalties you have already suffered remain in place and bonuses still accrue
So, how about that? Is that better for Toughness? How about the two new feats?