Jeri, I think the reason Song Challeneges are usually in a lighter vein is that it is easier to get a group to write on a common theme if it is humorous; serious songs--those that draw on the feelings--are much more subjective things.However you will find a lot of "serious songs" in the Songbook, and I am sure the Book's Keeper would be thrilled to have you add some.
Send 'em along if you like.
A