I think that his mother is a Tropius, that's why his surname is Gropius. Ghetsis + Tropius! And he has green hair and Tropius has got green thingies on its head too! :o
......Great.....Another Rule 34 image popped into my head, and Ghetsis is a bit more twisted than i thought.
Anyways, I actually believe the whole N is Black's brother. Here is the thing though: Black's mother saw how wicked Ghetsis was after N was born, and decided to run away from Team Plasma in an effort to spare Black from N's dismal future. She raised Black on her own, keeping him in the least developed area of Unova, and feared that Plasma may come for him one day, which is why he started his adventure so late. After hearing that Plasma was near by, she rushed after Black to make sure Ghetsis had not taken him, only to see that he did not recognize his own son standing so near. Relieved, she rushed to by Black a pair of running shoes, more as a precaution in case someone from Plasma did recognize Black, and rushed them to him.
Hence why she is right behind you with the shoes. Zekrom and Reshiram merely reinforce my idea that this is what transpires, simply because the princes were brothers, and had similar feelings but separate ideals.
As for N being a Zoroark, impossible. Pokemon cannot catch other Pokemon and cannot talk, both of which N does.
Anthea and Concordia are both Aunts of N. They knew of Black's birth, which is why they urge you not to harm him, and tell you his story. Black's mother made them both promise to never speak a word to N or Black of their siblings. This was to keep Black safe, and N secure in the fact that his mother loved him very much, despite the fact that she could not be around.
N is not adopted, because Ghetsis even acknowledges that he is N's father, but had only used him for his own gain. He does not say he is N's father, but he acknowledges the fact several times throughout the game.
As for N's child, I believe that he eventually falls for Bianca. She represents things in life he sought to protect: innocence, naivety, selflessness, and, to some degree, helplessness. Seeing her love for Pokemon only makes her seem even more perfect to him. Their child is most likely a boy with blond hair, green eyes, and a deep love for Pokemon. They move back to Bianca's home town, where N is reunited with his mother, and she explains everything to him.
God.....I put too much thought into this.