It's currently rumored that Richard Rider is set to appear in the post-credit scene for Guardians of the Galaxy.
Now a second rumor is stating that Nathan Fillion may cameo as Richard Rider Nova (via Latino Review).
So it's possible Nathan Fillion could be featured in the post-credit scene as Nova.
Nathan Fillion already has been a fan-favorite choice to play the role of Star-Lord and even voiced Green Lantern for various animated Warner Bros. movies.
"Guardians of the Galaxy" has an August 1, 2014 release.