@Fire Fist:
How about Mist Gun being his father? After all, he just "happened" to show up for this arc, and we know basically jack about him except he doesn't like showing his face to anyone but Makarov.
And Luxus has claimed to have seen it before. So Mistgun being his dad would make sense.
Maybe Mistgun is going to go spank Luxus for being a bad boy then.
Though I am leaning towards that Luxus' parents or at least one of them is dead. And the other doesn't really have anything to do with him. Maybe he only knew one parent. But from the looks of it. Marakov raised Luxus.
Plus If Marakov's child was still alive it would be more than likely that they would be the next in line for master of the guild instead of Luxus.