Why make rich from xandar?
Exactly, it really detracts from the character and the only thing that might add is make Xandar's destruction more personal but him not being from Xandar still allows for that event to be just as traumatic.
But most of the arguments I've seen is from people that don't like or haven't read his pre-Annihilation stuff and just want Rich to be this space Clint Eastwood type of character without the Rider family and the stuff from Earth. I guess they think Earth detracts from the character, and Xandar would allow for him to be full cosmic? But it would really kill a lot of what makes him interesting.
But even in Annihilation and the DnA run we get moments that highlight Rich being a Terran, how space still surprises him, and how important Earth and his family are to him. Hell it's Earth being in danger that gets Rich to run off blindly at Annihilus in a one desperate attempt to kill him. The United Front dissolved and lost the war but the Annihilation wave is going to kill Rich's world and his family. Not even Peter Quill gets that desperate about the situation (Peter really has little to no love for Earth it's were his mother died and it's a place he desperately wanted to leave). As a matter in fact Peter thinks going after Annihilus is a dumb suicidal plan and when he finds out the wave is going to Earth he was fine with leaving the problem to the Earth heroes. It's Rich going that makes him go, not Earth.
After he ends up in Kvatch he dreams of his past as a teen and of a future where he's an old man and has to take on the Phalanx which took over Earth and he has to fight his planet, its heroes, and his loved ones on Earth.
Then there's the Rider family, cutting them off would be bad because they shape Rich as a character and a lot of his positive and negative traits are in due to them. Rich's biggest fear is failing his family and his biggest motivator is to protect them.
His family also has a big presence in DnA's run. Just see how they recieve him after he comes home from the Annihilation War, how he thinks about them in the Nova annual, and anything from the Secret Invasion tie-ins and after especislly with Robert Rider who becomes a supporting character for the duration of that run. I could write all day about the Rider family they have so many flaws and good things about them and all of it impacts and marks Rich both in positive and negative ways.
But yeah if they are going to have Rich in the MCU he needs to be from Earth and have his family feature in some capacity. I have talked about this before but the Nova who could benefit from being half Xandarian (and emphasis on half because his Earth and Mexican sides are important too) is Sam Alexander. Because legacy and heritage are two aspects that are important to Sam's character and are featured in his comics (he inherits the Black Nova helmet from his father and on his mother's side we are shown that his Mexican traditions and family are very important to him) and it would also give him a connection to Xandar. Plus there's some stuff in Loeb's comic that suggets Jesse could be Xandarian or at the least there's a lot about Jesse that Sam doesn't know.
