I would think for a writing portfolio you are going to want to have different samples of your work...depending on the type of writing you do/enjoy the most, you will most likely have that be the strong point in selected examples. If you are able, try to have different levels of your work, for example, have a piece of writing you have done a couple of years ago, then a piece that is more recent in whatever genre you write in. That way you can show depth and growth in your work, but I also would not put a lot of work that is not your best in there either.
I would think having other examples would be good, but not everyone enjoys writing poetry, nonfiction, fiction, sci fi, romance, script, play, professional (business letter, resume example), etc. more.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.