One of the best ways I've found for finding out what's hot on the net is to browse webmaster forum marketplaces.

Reading through the listings you can gain a massive amount of intelligence of potential markets. You can often find out what have been successful traffic sources, promotion techniques, income sources and more.

Another idea is to try to identify emerging trends (eg. if you noticed arcade sites becoming popular) and then try to cater for webmasters in that space.