This depends entirely on your audience, your own personal tastes, and the convention you’re going to. If you are selling at a convention that doesn’t allow fan art, then obviously you shouldn’t bring any. If the convention has a specific focus, like Yaoi-Con, then the convention attendees are going to be looking for specific themes and subjects.
That part’s fairly obvious – if you bring a portfolio of nothing but girls in bikinis to Yaoi-Con, you shouldn’t be surprised when your sales are slow – but when it comes to a convention with a wide variety of attendees, it’s a bit more complicated, and this is where your research will come in handy. You’ll learn what sorts of subjects that you do sell. And this is dependent on personal style, too: my pictures of women don’t sell very well, but other artists’ pictures do. I can’t point at what in my style appeals to those looking to buy pictures of men and doesn’t appeal to those looking to buy pictures of women, but it’s somewhere in there.
Check the policies of the convention you are attending before committing to something. Some conventions do not allow fan art. Others do not allow prints, just originals. Others may allow limited quantities of fan art prints. Still others may prefer that you stick with art and not sell craft-type items. Follow the rules so you don’t get kicked out.
Keep in mind what your strengths as an artist are. If you excel at drawing shiny pieces of mecha and suck at drawing pictures of humans, you’ll probably sell more pictures with mecha in them. If you’re better at graphic design and bad at anatomy, focus on that. If you’re good at drawing simple cartoon shapes, maybe you shouldn’t waste too much time trying to draw elaborate Gothic Lolita styles no matter how much you like them.
Don’t let someone convince you to draw X because they say it’ll sell better – no point in putting weaker work on the table.
That being said, I’m going to overgeneralize again and say there’s two types of art buyers: the crowd that doesn’t care that much about art skills and technical ability, and the crowd that does. Both groups are looking for art that speaks to them emotionally, that depicts characters and pairings they love, with colors and shapes that attract them.
Those who don’t care much about art skills and technical ability tend to be younger, often care less about the artist’s skills, usually spend less money on each piece, tend to equate lots of detail in a piece with good art, and may prefer fan art over original art.
Those who do care about skills and technical ability are more discerning, look more critically at an artist’s skills, have more of an appreciation for graphic design and negative space, and may choose either fan or original artwork. However, they also tend to budget more for each piece of art, and may turn into collectors of your work, seeking you out each year to see what new things you have.
You can try to appeal to both groups, or you can pick one and specialize – the first group spends less, but there’s more of them, so it tends to work out evenly in the long run. So don’t let worries about mistakes in your art stop you: there will be people who care more about the subject of the work than the skills involved in making it.