I would assume that they would be Res or Def focused, with a smattering of HP/debuffing abilities and could be slotted into either Res/Def/Reg-heal for armors.
So your First ATO armor slot would be slanted to Res with some regen/heal/debuff (and some resist debuff powers), while your second armor ATO would be slanted to +DEF with regen/heal/debuff with resist debuff.
Perhaps to make it a real choice, you can only do one or the other.
And since everyone gets some sort of armor (ancillary or patron pools) you could actually make a version of them for every ATO. Probably slightly generic, but with tweaks to focus on AT deficiencies. Slight Damage buff for tankers/defenders, higher mez resistance for ATs that don't need damage buffs, etc.