
BadBird |

Orc and Half-Orc have some stuff that builds off it, ending with a feat that makes it harder to confirm a critical against you (Deathless Zealot).
It works well with Barbarian Rage, since then your Barbarian can't die from knocked-out > Rage ends > CON drops > dead meat. 'Course, Raging Vitality does that too.
The Unbreakable Fighter gets Endurance and Diehard as bonus level 1 feats, which makes it a pretty spiffy way to get Diehard and Fighter proficiency with a single level grab.

SodiumTelluride |

Not feat suggestions, but build suggestions. I once had a 3.5 knight / Dwarven Defender (a knight was basically a cavalier with a d12 HD and no mount; Dwarven Defender was just Stalwart Defender but dwarves-only). By level 12-- the highest he got-- he had very high AC from his armor and tower shield, DR from his Defender levels, tons of hit points, and the few times he did drop below 0 he could still act normally. VERY hard to kill-- survived a whole campaign despite being always at the front and with a poorly-played Mystic Theurge as the party's only healer.
My advice: you're taking Die-Hard because you want to make it Hard to Die. So make it even harder. Armored Hulk barbarian levels, Stalwart Defender levels, the works.