There is no single genre that piques everyone's interest, just as not all stories within a genre pique the interest of readers in that genre. People who tend to read mysteries will by nature simply miss the release of most or all romance stories, but if you switch to writing mysteries you will probably lose a good number of your romance readers.
You could try writing a novel that spanned all genres, which might be an interesting exercise, but even that book would not appeal to everyone. Many habitual readers of a genre prefer things to fit within the mold somewhat, and for variation to occur somewhat within the confines of the limits of that genre. That's not to say that there aren't genre-spanning works that have been successful, but in most stories one genre predominates.
This is partly due to how most fiction has traditionally come to the attention of the average reader. Traditional publishing has been categorized into genres for quite some time, and when a publishing house considers whether to take on a new book for publication, they try to analyze whether it will appeal to fans of the genre. They know that when people go to a bookstore, for instance, the romance readers head for the romance section, and so on. This categorization occurs on book-selling sites and in book-of-the-month clubs as well. The categorization extends to magazines and a good many online sources of fiction as well. It's almost inescapable.
The mere fact of this categorization means that a good many readers wind up self-selecting by the time they're exposed to a book. Thus if you were to write even a successful romance novel, it would simply never come to the attention of most mystery or sci-fi readers.
Nor if you wrote an intentionally genre-catchall novel or short story would I expect it to be treated as more than an oddity. Would you want to read a romance-western-space-opera-historical-drama-comedy-horror-action-adventure? (Hmm... that actually sounds kind of interesting in a silly way, especially if you consider stripping out one or two of the genres.) Even if so, how many would? Do you think it would qualify as Great Literature, and appeal to its readers? Note also that some types of stories are simply incompatible, even within a single genre, for example space operas and "hard" science fiction.
(Note that there have been many authors who have spanned genres, but they tend to show variation within their entire body of work and not to be evenly spread between genres. An example is Kurt Vonnegut, who wrote some science fiction but who is not primarily remembered as a sci-fi author. Gabriel Garcia Marquez is another possible example that comes to mind, and in general many authors who adopt fantastic or magical-realist elements within their works might seem to straddle genres. However, here I think there is an additional division between serious literature and what's often called genre fiction, which is written to intentionally fit within a genre.)
Thus attempting to write genre-spanning genre fiction is not the answer, especially as that phrase likely qualifies as an oxymoron. The real thrust of your question seems to be that you're dissatisfied with your current readership. There are two main areas where you can more usefully focus your efforts: increasing the quality of your writing, and marketing the results. The former is far too broad a general topic for the focused question-and-answer format of this site. There are many books available on the craft of writing, even whole series dealing with different subtopics in turn (characters, plot, etc.). I'd read these first.
Aspects of the Novel, by E.M. Forster
On Writing, by Stephen King
Telling Lies for Fun & Profit: A Manual for Fiction Writers, by Lawrence Block
Bird by Bird, by Anne Lamott
Regarding marketing, I'm afraid that you are going to run up against some hard facts of the modern writer's life. While e-publishing has enabled many new writers to theoretically reach a wide audience, the reality is more depressing. Amazon and similar sites are so choked with ebook-only releases by unknown writers that potential readers, the ones that don't instantly head for the Stephen King and Jodi Picoult releases, are extremely scattered. Thus even if you were to write a marvel of a romance novel or anything else, you'd be foolish to expect it to coast to bestseller status just on the basis of online reader reviews. Fan fiction sites and other forms of cheap or free online forms of electronic fiction self-publishing fall prey to the same mass obscurity.
Thus if you want to reach a mass audience, you're probably looking at dealing with traditional publishers (Writer's Market is a good resource for targeting those). Unfortunately, the outlook has become considerably more bleak in the last several decades. Market forces set in motion by the wide availability of computers and the internet have made it tougher to become a successful fiction writer, by driving the ongoing demise of paper as well as greatly increasing the level of competition. NaNoWriMo and computer use in general have resulted in a massive increase in the number of novels written and submitted to publishers as well, making it harder than ever for high-quality novels to stand out and be accepted for publication. At one time magazines were a relatively easy way to break into professional writing, but the market for print magazines has diminished. Surviving magazines tend to pay less for short fiction, at the same time as they tend to have a much larger volume of submissions than in the past. There are number of online literary reviews and similar edited sites today that accept submissions from new writers, many of which sites do not offer pay for new work but merely the opportunity to publish on their site, but readership of such sites is not widespread, and publishing a particular story in such a fashion could greatly restrict chances at getting a traditional publisher to consider it later. Getting published on such sites seems mainly useful for padding one's resume or cover letter when submitting a work to a traditional publisher.
Finally, stay away from vanity presses. Money should always flow toward, not away from, an author.