Fillion did cameo in Gunn's Guardians of the Galaxy and is known for voicing Green Lantern in the DC animated universes.
A fan at Wizard World Chicago asked Fillion if he could play any character in either DC or Marvel, who would it be?
"I think I could take a pretty good crack at Booster Gold," Fillion told the crowd, who were in attendance for a Firefly panel. “That’s kind of my niche - show-offy, vain. I think I could handle that - not too bright. I think Ambush Bug; little bit off his rocker. You never see my face. But I think Greatest American Hero is do for a reboot.”
Fillion then questioned Summer Glau about what she knows about super heroes, with Glau responding, "I like She-Ra."
Fillion replied, "You are totally a princess of power."
Adam Baldwin chimed in stating maybe he could play Green Lantern, with Fillion shaking his head no.
"I don't know," Baldwin smiled, and they all laughed with Fillion stating Green Lantern has already been taken.
Regarding Booster Gold, a while back there was talk about a SyFy series in development, but really nothing has been mentioned in a couple years.
Interestingly enough, a Greatest American Hero remake is underway.
You can watch the video below for more from Nathan Fillion talking super heroes.