I like the idea to have an unwanted secondary effect for each damage type (like fire makes the target faster).
Cold gives small armor, i like.
What about acid ? It could damage the melee attacker by a small amount.
EMP could be friendly-fire, so you can get fried too.
About combo, it can be a lot.
fire then cold = cancel fire
fire then EMP = disable random power for X seconds
fire then acid = AoE explosion
cold then fire = cancel cold
cold then EMP = guaranty stun (short duration, then make the target immune for some time)
cold then acid = cancel armor, keep slow
EMP then fire = greater damage
EMP then cold = greater slow
EMP then acid = greater effect
acid then fire = damage also ranged attacker
acid then cold = slow in AoE including allies
acid then EMP = knockdown ?
If a target gets fire then cold then fire then acid... i think it should only count the two last effects, so you can specialize in some combo, and as a target to don't rage too much because you are always under an effect.