Like most genres, the plot is the most important thing you can plan. If you can come up with an amazing idea then you can gain hundreds, maybe even thousands of reads extremely quickly. But if you have a rubbish idea then it doesn't matter how well you write your story, because a rubbish idea doesn't attract people to read a book.
The audience for this genre is large, but finding readers may cause some issue. I don't know many accounts that just read Mystery/Thriller, people tend to read Horror, SciFi as well. So if you plot can bring in aspects of other genres (such as Horror) you can get a wider reader base.
Don't hold back on the gory side of your plot- trust me, nothing is too gory for this site. Stick with your gut instinct when it comes to planning, use any good ideas that you have.
To find inspiration you can look around the Mystery/ Thriller page to see what kind of things people write about but whatever you do - DON'T PLAGIARISE! That is a very important piece of advice.
So get thinking of your idea!
- E