Pick something you think would do good, or pick a niche.
I mainly do work in ecommerce, so I create modules to extend shopping cart functionalities. Because ecommerce is about directly making money, then I can sell modules for fairly high prices ($60+ each no problem) with relatively little code. The only hurdle comes from providing support. Conflicts with other modules, cart upgrades, different server configurations etc.
You can make anything sell well if its actually worth it. The hardest to break into would be actual shopping carts (competing with zencart, oscommerce, magento etc) and forums (competing with vbulletin, smf and ipb.)
Just ask around and see what people want to optimize and try to create a plan that would do that. Bonus if you can add in other related functionality.