Re: writing to a hot genre, keep a few things in mind.
What's published today was bought by publishers 2 years ago on average for fiction. Unless you can find out what genres or subgenres they're excited about now for publication in the future, this could easily be a moot point for what you're writing now.
How long will it take you to write it, polish it, start submitting it? Will you also include getting it critiqued? All this takes time. By the time you're submitting it, the editors looking to buy manuscripts might be looking for something else, a different genre than what was hot when you started writing it.
Unless you have inside info or know folks in publishing houses who will tell you otherwise, for all you know, the editors now have plenty of mss for the current hot genres, thank you very much, and don't need to see more. They like striking while the iron is hot and milking trends (think DaVinci code and the similar plotlines published since), they do understand the market can reach a saturation point and readers will get bored. Editors/publishers also want to find the next new hot thing. You don't want them thinking, "Oh, no, not another of those. Can't people come up with anything fresh?"
And genre is blurring these days, anyway, Mysteries are morphing into suspense or thrillers, romances are mixing with science fiction or suspense, westerns are mutating into family dramas out west.
Re: being able to write it in any genre:
Ah, but can you? Wanting to doesn't mean you can approach each with the same level of skill or expertise. Are you more familiar with one genre over others? Genres tend to come with built-in elements that readers of those genres have come to expect.
Sometimes a writer not comfortable enough in a genre might not give it his or her full emotional attention and it can show in the writing. The writer could sabotage his/her efforts, even subconsciously. Many writers I've encountered online have talked about the story being the dictator of genre. It's one thing to start with a theme, but only you can decide how best to tell the story. What gets you the most jazzed when you think about it?
Publishers also tend to label books whatever they think will sell them. So a lot of genre titles have been called "Fiction" these days to get past the science fiction label. The most interest genre is for the writer, IMO, is to determine which publisher to submit the ms to, ie to start with the ones that publish books similar to what you've written, or at least, the most similar.
Bottom line advice is to write the story. Sit down and do a comparison chart if you have to of the story as told in a variety of genres and see what works best for you. If you don't feel you've picked the best genre for the story, you'll have a heckuva time trying to convince an editor that you did.