While i understand Avaron's reservations if you want to prove your chops as a writer, start small do an interesting character piece with whatever lights your candle, just keep it short and sweet so that you can get it done in a reasonable amount of time and build some rep.
If you need a little nudge in terms of content maybe try a fairly realistic depiction of polygamy, focus on personality and character, explain why the marital unit are attracted to each other/work as a group. Alternatively you could go for a morality tale, something involving a man in power (police chief, politician ect.) using his power for self service or the common good, how they resist or fall to temptation and how the main character reacts to how he changes across the course of the piece.
Now this could sound harsh but no matter how good you are as a writer if you can't prove it people will think twice about paying you or commissioning you, you need proof, a small body of work would be good IMO. I would suggest that if you want to be paid as a writer perhaps starting out by writing some short erotica and posting it up on something like Literotica would be a decent place to start, it at least gives you something to show people, not to mention having a finished piece of work behind you feels amazing.
Alternatively if you're not after being paid immediately try going in as an editor/co-writer for an non-English speaker who is making games and do the work for free to begin with, they get to use your English skills and you get to prove your chops, heck if the game becomes successful you can start talking about perhaps taking a cut.
I'm looking forward to what you come up with, good luck.