What we know: Nathan Fillion has a small fun cameo in Guardians of the Galaxy.
It's already been debunked by James Gunn that Fillion will not be playing Nova.
Following that bit of info, just about the entire internet speculated Fillion will be voicing the fan-favorite Russian dog from the comics, Cosmo.
However, Gunn posted the following:
You fan journalists need new sources.— James Gunn (@JamesGunn) June 24, 2014
The good news is that Gunn didn't specifically state the tweet is regarding Fillion and Cosmo.
He also didn't say the speculation was wrong.
So, if Gunn's tweet holds true regarding no Fillion as Cosmo, speculation than leads us to possibly Fillion playing Adam Warlock.
It's thought Warlock's cocoon was featured in the Thor 2 post-credit scene.
"Guardians of the Galaxy" has an August 1, 2014 release.