I have always felt that the perfect antagonist for the Originals would be one of their siblings and not their parents because so far it's been "Yay we are working through are issues to become a real good family!" and I'd like one of their siblings to pop back in say "Hey jack asses! Did you forget about me?"
Ideally I think Henrik Mikaelson would be the ideal candidate for this since it's unlikely that Kol and Finn could return. Especialy since he died young and in Klaus's arms I can see him coming back and posing as a human maybe running for Mayor of New Orleans (since Klaus killed the old one) and while pretending to make peace with all the factions it comes out that Henrik was really plotting to wipe out the witch's and run his three siblings out of the city.
Ideally the truth would be exposed after Elijah connects the Quarter's new Mayor to a set of murders occur in Mystic Falls (Jeremy and Matt) Then when confronted and exposed as their long lost brother he justifies his actions against his family by screaming "Don't give me that always and forever crap! I was left alone in this world with no one while you all got to be together at the expense of Kol and Finn's happiness. You Klaus did nothing to avenge Kol or Finn and you Rebekah you let Finn's killer into your'e bed I should kill you for that alone! And you Elijah you did nothing to stop Klaus from tormenting our siblings! I killed those two insects Jeremy and Matt and when I was done I left left Mystic Falls completely ravaged something our father would have done because he was a real man and a real man takes revenge for his family!"
I really want this to happen because the idea of one their siblings coming back to get them and not only trying to take everything they have but also praising Mikael sounds sweet. Especially considering how much they hated Mikael.