I suppose you could. And people have done so already. I can't see how or why anyone would stop you. I could think of a few reasons why I think it could be a bad idea. One reason would be that if you look at all the threads, they contain a lot of information, and that is going to grow, we will have more and more material as we go on, and it's going to be increasingly important to have some kind of structure to make it possible to find stuff. A new member of this community will have a hard time catching up otherwise. So far, we haven't got a section for season three. So where would you post the suggestions?
I guess you could start general threads like the one on Elves, but for Dwarves or Men. Or add ideas in the Elves thread. But a specific season three thread seems a bit problematic to me. Others might think differently, I imagine.
I think, in agreement with Haakon, that it wouldn't be a huge deal, but it would be slightly less organized. Putting it in a Elves, Men, or Dwarves (depending on the character), thread would at least maintain a level of organizational integrity.