By signaling that you are capable of putting your own spin on someone else's work, you may qualify yourself for a shot at writing one of the many releases that adapt novels, comic books, foreign films or board games. A strong adaptation plus strong original material could give you an advantage over someone with just strong original material.

The best known recent case of someone writing an adaptation of copyrighted work is Emily Fox's Blockhead, which asked what happened to the Peanuts gang when they grew up. It could never be purchased or made, but it won the writer a number of meetings and a first job in TV.

If I have understood things correctly you are intitled to write whatever you like, BUT you have to call it a writing sample and not try to sell it.

It can be useful to have something well written to show up when you want writing assignments. BUT it takes a lot of efforts to write something you aren't likely ever to sale.