From an interview with Joss Whedon:

"The reason I feel strongly about doing a sequel is [when] you make a movie, there are so many things, especially because it was based on a series that was supposed to run for so long—there are so many things that you want to do that you're unable to do or discover in the process that you want to explore, the most important of which is that Adam Baldwin really wants to wear a kilt."

...I just went to a very happy mental place right now. I'd never considered it before and I don't want to see it written into the script for the fangirls' sake, but gravy, he would look really, really good in a kilt.

Ah, Joss. Shirtless Simon, nekkid Spike... You have no shame, and we love you for it.

Mmm, shirtless Simon.