If I remember correctly, James Gunn did not want to have "Nova" in Guardians for two reasons, 1. He wanted Quill to be the only human out in space, which would make him unique (which actually does not work because Quill is not all human and the Guardians look human anyway, mostly, and 2. He wanted the heroes to be underdogs who come up against supremely more powerful threats. Well, in the sequel MAKE THE FIRST NOVA THE THREAT!
Have Rhoman Dey (who I would want re-cast) take hold of the power gem so he can protect Xandar from some big threat. He becomes the first super powered Nova Centurion, who goes a bit crazy. The Guardians go to "rescue" him, but end up having to fight him. Eventually, they learn how to subdue him and how to give a tiny bit of the Nova Force to individuals. Maybe a computer (World Mind) would be used for this task.