When all else fails and the universe has no one left to call for help, the Heralds of Revenge come from the the light and dark to work together to take down the greatest of threats.
My Captain: Darth Vader
The reason I choose Darth Vader as my captain is because he has been both good and evil, and his sole reason for becoming more powerful is to seek revenge towards those who hurt his family. He uses the dark side of the force to crush all that stands in his wake. Also he is for all intesive pourposes the leader of the entire Galactic Empire.
My first pick: Natsu Dragneel
I choose Natsu Dragneel as my first round pick, because no matter the odds and no matter what stands before him, he will over come it and defeat those who wish to destroy all the things he loves.
My second pick: Ghost Rider (The Spirit of Vengeance)
My reason for choosing Ghost Rider is his ability to tell the Devil to shove it where the sun don't shine and because he is a total bad ass, and him and Natsu would be a great team up.
My Third pick: Ichigo Kurosaki
My reason to pick Ichigo for my third pick is that he would be able to balance out the light to dark ratio being that he is both.
He is also like Natsu in the way that if he has someone or something he loves to protect he will fight to his last breath protecting what he loves.
Why I believe this would be the ultimate team up to stop any threat in any and all universes.
1) Darth Vader can use the dark side and control of the dark side is uncanny. He also has plenty of experience leading people into battle. Natsu and Ichigo may work with teams but they tend to fight alone and thou they maybe stubborn hard heads, they can work together to take down a serious threat. Ghost Rider would not want to work with Vader because of the evils he has done, but if the opponents are worse then Vader, GR would take care of 'Priorities'. Also as I said b4, if Natsu does get in over his head he can also eat GR Hell Fire to regain his strength.
2) They all have one thing in common, they will not give up till they have given everything they have and somemore.
3) So in conclusion, I believe this team would be great, but at first there would be A LOT of bumbing heads.
P.S. Natsu would also make sure they all have the necessary physical strength to match his or exceed it.