I was wondering whether it would be possible to easily create a drop down menu of categories, and due to the large amount of choices, have the whole page beneath it move down to make room.
You can create one overlapping the content of website. Just click the forum on digital point forum and you will get an idea.
It would be possible. Maybe a block would come down with all your categories in it. I think it might have to use Javascript but then again CSS might work just the same. Google drop down menus and see if you can find anything remotely like what you want