A Necromorph costume would be pretty sweet, especially if you, say, jumped out of an air conditioning vent at a Con or something (not on the ceiling of course).
I actually don't plan on doing spartan ATM, but I have several costumes in mind for when I'm done with my odst...and after I save some cus I'm emptying my piggy bank making this suit...
Armor:
COG Armor (Gears of War)
Unicorn Jabu (Saint Sieya - AKA Knights of the Zodiac)
I built myself a quasi - Warrior prestige armor set from guild wars for dragoncon, and then wussed out and didn't take it! (60$ checking bags on a plane... sooo poor) I think I'm going to wear it for halloween tho. I want to make armor from Aion, but goddamn.
Defintitely some Megaman X armour variants. Like the Shadow armour or Falcon armour.
How about the OMEGA suit from Shadow Complex. Looks pretty simple short of finding a way to make the helmet work so you can see out of it.
And I know it's not out yet, but what about that heavy weapons variant Mjolnir suit from the Halo:Legends trailer? That would be awesome, though cumbersome.