I personally think that later in the game you'll have to fight Angeal and Zack will end up killing him. Reason being because Angeal and Genesis leave SOLDIER together and Sephiorth suggests that Angeal has gone traitor. There's also the scene where Genesis and Angeal are fighting Sephiroth together. Another scene shows Angeal advancing on Zack as if ready to fight. It just seems obvious to me.

The whole Genesis thing has got me confused also. In the scene at the end of DoC Genesis says "We have much work to do, my brother." He is holding Weiss in his hands while he says it however I think he's actually referring to Sephiroth. Makes more sense to me that way.

I know that Genesis is the prototype of Sephiroth so that could explain why both he and Sephiroth have on black wing however, Angeal has a white wing which leads me to believe that Hojo worked on him in some way different to S and G. I'm not even sure if Hojo actually has anything to do with his white wing actually it just seems like the only possible explaination at this point.

Let me know what you guys think.

Hojo may have worked on them both, because Genesis had a disease and needed new DNA or something and Hojo seems to be the man for that kinda thing.

Extreme CC spoilers below.

The origins of Angeal and Genesis

Genesis and Angeal are both results of project G(which stands for Gillian). Project G was headed by proffesor Hollander, whose goal was to make an ancient before proffesor Gast and Hojo did.(remeber at the time it was thought Jenova was an ancient). The project started by injecting Jenova cells into a woman named Gillian Hewley. After this they took one of her egg cells, put it into another woman's womb and allowed it to be fertilized and grow. Angeal was made by Hollander having relations with Gillian, and the child was allowed to grow in her womb. Genesis and Angeal both result from this project. Apparently though, there is a flaw in the project because both Genesis and Angeal deteriorate, though Angeal at a much slower rate because he was the best result of project G.

Genesis was initially deemed a failure though because he was born a normal kid. He joined SOLDIER because of Sephiroth, who was someone around his age who had already achieved so much.

About your theory of Zack fighting Angeal.

Zack does end up killing Angeal. Angeal has begun to degenerate into a monster, and he wants to die while he still has his humanity left so he asks Zack to fight him to the death.