Neil Patrick Harris has recently said that a Dr. Horrible sequel couldn't happen any time soon. Not only is he interested in returning but Nathan Fillion (who played Captain Hammer) is also looking forward to it. According to Sci Fi Wire:
"It's actually in one of the songs on the Dr. Horrible DVD commentary music."Let's hope it happens soon.
But Fillion said a sequel won't happen right away. Fillion is starting a new TV series, and Whedon is working on his own show, Fox's upcoming SF series Dollhouse. "He's busy with Dollhouse right now, and I'm a little bit busy with Castle right now, so not in the near future, no," Fillion said.