Its still possible to write original game stories, but I'm actually wondering this - how does one write a game that stands above the rest originality wise? One can borrow from successful games to make their own. One can experiment, but it could be unsuccessful. That said, I believe originality is one form or another is still there for game writing and design. It just appears in more subtle ways. I like thinking about those cool ideas to write as a game, but its about making them original or capturing people's attention that is the tough part. Hmmm...how do you see this?

It's said that there are only seven original story plots.

https://en.wikipedia.org/wiki/The_Seven_Basic_Plots

http://www.nytimes.com/2005/04/15/books/the-plot-thins-or-are-no-stories-new.html

So don't worry about being original. Just, you know, be original. Nobody can tell you how. Just be you.

You don't have to be completely original. Just write something that affects the player's (or reader's) emotions. Introduce a character that the player "works with" or otherwise creates an attachment to, and then kill them off. Give them a weapon after a mission or quest that reflects or reminds them of the mission results. Sometimes it's having the player hang around an area, helping the inhabitants or saving them from something, then returning them back to that place much later (and much stronger) to find that all their work has been undone. You want the player to gain ownership or kinship with people, places or things, and then maybe even twist it around. 

Games allow boring stories to become great, for example: The hero fights a dragon and saves the village.

It's a very boring and unoriginal story. The player fighting the dragon get's to experience it and to them it's a much more interesting story. It's why so may games can get away with bad stories as experiencing them allows for much more depth.

The best game stories are ones that delve deep into the world they're in, but in a meaningful way. They also have characters that seem like stereotypes, but as we get to know them more, their own originality comes out.

So in a way, trying to be original just means making sure your world, characters, and plot come in a way that people will enjoy.