I agree that Sub-Zero should be there but he is not the romance type. He has been trained since he was a toddler to reject emotions which are deemed unnecessary. As he has no use for love, only to be an assassin. Besides, Loki seems to be the handsome lady's man there.
Loki is not ice-powered. He is only cold-resistant because he's a half-Frost Giant. So I have no idea why he's there instead of Sub-Zero (not that Sub-Zero would care about Elsa as more than a friend since he prefers hate over love (but he is an honourable man, which means that he respects people)).