I don’t know which types are in highest demand, but I would definitely say “Find what you do best, then do your best”.  We don’t need to flood the market shoddy content.  I don’t mean to imply that there is shoddy content on this site, but if I decided to create mermaids, because they seemed popular this year, I doubt if I could do a good job at it.  I have never created a model, and my first attempt should be something small, simple, useful, and not already available.  Instead of a full mermaid, I might just try to create a seashell bra or a simple underwater landscape.
My first (and only) upload to ShareCG is a computer program, because that is where my strengths are. 

Another factor is “What are you interested in?”  If you want to do a good job, it will take you many hours, days, and possibly months to get it perfect.  Do you really want to stare at an ugly two-headed troll for all that time?  If the answer is yes, go for it.  If not, choose something else. 

If you want to be a leader of the trend, instead of just lost in the crowd, keep an eye out for what is popular in the real world.  Popular movies and cartoons can inspire creativity.  I’m not suggesting you create a copy of The Simpsons, because they are very protective of their characters.  Harry potter is still very popular, and you can create characters similar to them (either from book or movie), or some of the creatures the meet.

When considering seasonal characters, you need to think several months in advance.  If I need a Santa Clause, I will want to download it in November, or even October.  That means that you would have to start creating during the summer or even the spring. 

Versatility is also good.  If an outfit has morphs, textures, or pose files so that it can be several different outfits, or fit several different characters, people will enjoy that.  If you create a scene with walls, floor, furniture, and knick-knacks, then you should try to keep it as separate objects.  This way they can rearrange the pieces, adding pieces from elsewhere, and create whole new scenes.

Outfits and accessories for new free characters should also be popular.  When DAZ gave Victory 4.2 away, there were a lot of people with a new character who had nothing to wear.

One of the best ways of figuring out what other people might like, is to figure out what you would really like to find already made.  When you cannot find it, create it.

              -David