Lucius and James:
I’ve got two different ideas on this one; I would either like this to be a hidden relationship where they’re both rather ashamed of what they’re doing, or else one that’s platonic yet forces them to revolve around each other. Some various ideas I’ve got:
Lucius stops James from bullying Snape, and Snape runs away, leaving the two boys together. Lucius and James get into a fight and things go from rage-filled to raunchy…and then they keep going back to each other for angry, night-time trysts despite hating each other and everything the other stands for.
Lucius and James know each other from child-hood (both being from pureblooded families) and Lucius continuously tries to turn James to the dark side. Despite his relationship with Lily, James doesn’t want to see Lucius falling into the Dark Lord’s rabbit hole and keeps trying to get him not the join the Death Eaters.
A completely platonic relationship, at least in the sense that the plot would revolve around their interactions with each other (James as the leader of the Marauders, and Lucius as the leader of his own little gang of Slytherins)