A big part of Sam's backstory is that his father was a Nova. That causes problems since the audience has never been introduced to the superpowered Nova Corp and GOTG indicated that the Nova Corp didn't have the ability yet. If they used Sam they would have to not only explain the change but show that it happened long enough ago for Sam's father to have joined and left. since the first GOTG.
With Rider the story is easier, less things would need to be changed, Rhomann Dey (who appeared in GOTG) crashes on Earth and passes on the Nova Corp Helmet before dying, Rider then goes on to become the first Human member. Its basically the same story as Hal Jordan becoming the Green Lantern.
The main thing would be explaining them going from no powered Nova Corp to having it in the new film. For me the simplest would be that the technology was created through examination of the Power Stone although there plot hole in GOTG. Ronan's ship the Dark Aster was supposed to be a Kree warship, if Xandar had just ended a war they had fought with the Kree for centuries you would think that they would have plenty of experience fighting them yet they were struggling to deal with that a single warship. Did they have bigger and better weapons elsewhere?