Well as long as you don't want us to actuualy write the 15 page paper for you...
Welcome to the site, Quarandy!
For me, Terry's writings are about depth more than humour. He writes about what it is like to be a human being and his characters are so very recognisably 'human' - even if they are dwarfs, trolls, vampires or any other denizen of the Discworld.
Terry's writing is a bit like an archeology dig. There may be a trace of something on the surface, but the really good stuff is buried. He doesn't spoon feed his readers, he just writes and the readers either get what he's talking about or they don't. I don't think I've ever heard Terry try and explain what his books are really about.
This sort of humanistic writing will never go out of style because humans really don't change very much. He's like Jonathan Swift who was writing around 300 years ago and yet his writing is still relevant because, as I say, people don't change much at all.
"Goodness is about what you do. Not what you pray to."