I don't remember any upgrade for the dark suit in Metroid Prime 2 other than the light suit. And the hazard shield would make the PED suit look more unique.
There are a few upgrades for it aesthetically, though they're more like accessories; the spider ball, the gravity boost, and the grapple beam. I think it'd be best without the grapple beam, given how it's used with the arm cannon, but with the gravity boost, due to Samus' jump using back jet-thingies.
I like your reasoning behind the hazard shield, I didn't even think about it before, and given how the team would have to change the shoulders' size, the hazard shield would look better bigger. that what about the corruption level? I think the corruption level should appear at either 10% or 25%, it's a nice grey-ish, anything else is too blue, and too similar to the Dark Suit recolour.
I'd be cool with either dark suit recolor -> phazon suit recolor and true dark suit or dark suit recolor -> true dark suit and PED suit with hazard shield.
I think the chance of the true dark suit completely replacing the recolour and adding a PED suit is not very high, since the team tries to replace as few Brawl palette swaps as they can get away with... But then again, if they did, there'd be an equal number of palette swaps as there are costumes,
Though I would like a true gravity suit recolour: More blue, bigger shoulders and more lights!
And the green colour from the Prime 2 Echoes Multiplayer!