Honestly, I think the biggest concern about whether or not to have a subforum is whether discussion of this particular topic is drowning out discussion of other topics. The worst thing to happen is to have a subforum and then find the regular forum is now dead.
Right now, superhero movies just happen to take up the bulk of our conversation here. There are several sci-fi movies coming out about now that I haven't seen any discussion of. If those were being drowned out, it would be different. I guess the question is whether those threads would see some light if there were a subforum. The other question is would there be more discussion of print media comics if there were a subforum.
Like I said, I'm in favor of one. There's just a lot of concerns any time you add something new.