All the starter survivors have a positive relationship, or at least a working one,
with their ally at the start.
I think it would be neat to have a survivor pair with two strong individuals, but they despise each-other from their lore. (Criminal/Cop, Bully/Victim, Estranged Family, Unfaithful Lovers)
Their rivalry could lead to them sabotaging each-other, or worse, unless they work through their problems with each-other.
Maybe one of their side-quests could have them respecting each-other by the end of it, and unlocking their full potential together.