Brainstorming and generating new ideas doesn't end after the final line is typed in your book. It doesn't end once global edits are completed with your editor. It isn't even done once the cover is revealed, the copyedits all polished off.
You just keep on brainstorming new ideas--for promotion, spreading word about your book.
One of my latest brainstorming sessions allowed me to put together the trailer for my upcoming YA, SPARK (due out May 17). I had a ton of fun with this one: