Actually, I'm with Jaguar on this one. Pick a topic you like and you think you're good at. Get modelling, refine your skills, look at what preforms, get better. You're much more likely to do well in a field that interests you, rather than chasing what you think might sell. And what sells for others, may not for you. And unless CGT fix their search, ranking and "quality" checking system, it doesn't matter, nothing you upload will sell.