Stone Giant as an execute was my initial idea as well, but it is a projectile, and that is bad. Especially since DKs are supposed to be in close range. That said, if the exe morph would be a melee skill, it'd be perfect.
Stam DKs have Minor Brutality and stam refresh through their passives, on top of the best anti-ranged tool and great synergy with poison. Venomous Claw and Poison injection are very, very good DoTs in PvP. Molten is amazing for ganks or just plain ranged burst. Yeah, stam DKs might not be god mode, but they are certainly a very strong and harmonic build thanks to great synergies.
Magicka DKs do need an execute, as they have barely anything uber going for them. Good AoE DoTs don't make them OP in boss fights. And every other class has an execute, so this isn't an exclusive thing, but more of a necessity for any build.
Sorcs don't need a spammable, destro staves just need a big buff. ZOS in their stubbornness just refuse to admit that in this game, where ranged combat does not exist thanks to gapclosers, the weaker damage output from staves is not justified by melees having to get in close with higher damage.
Temps are supposed to be healers, and they do that very well. Anything else they get from weapons or sets, so no need for built-in buffs other than healing.