I'm an optimist to the ninth power. I believe in the power of positive thinking and dreaming and goal setting, but there is a time and a place and a limit to what these things can do. You can dream about becoming as famous as Stephen King. You can set a goal to be a best-selling author by age twenty-two (I only have two months in that case!). You can envision the movie version of your book, what the cover art might look like, if it will ever be banned, and if the President will endorse it. That's all fine and dandy, as long as you don't let the dreaming take over the reality. Nathan Bransford has a good post about dreams and expectations.
Writing a book isn't a gimmick. It takes hard work, determination, patience, and a lot of disappointment. How can you ensure that you write a book worthy of being published, worthy of living up to your dreams?
It doesn't matter that vampires and zombies are the thing of last year. It doesn't matter if steampunk and mermaids are on the rise or that historical romance is slow. Trends don't matter. Don't write to trends. Don't not write to trends.
Write with passion. If you are passionate about vampires, then by all means write vampires. It doesn't matter that they're supposedly "dead." If you are passionate about zombies, or 18th century homoerotic romance, or alchemy, or middle school, or steampunk vampire unicorns, or whatever... write it. J.N. Duncan, who I interviewed last month, has a great post on his blog about writing to the market.
If you write passionately... if you write what you love and what you can't help but write, no matter what the market says about it, then you will write a book worth publishing. It will take time, and it will need polishing and editing, but that's just part of the industry. If you write passionately, someone will recognize that passion, and they will share that passion.
So put your passion on the page, and you'll create something worth reading.