I think one of the better ways to go is to get paid by a merchant(s) to develop an App they actually need and want, and are willing to pay for. That way your time is spent productively making money. If it turns out other merchants could benefit, then you have a nice head start on the App development process for all merchants and a more general version of your project.
Waiting around for a popularity contest is somewhat fruitless as there are a couple of things going on. Merchants are clamouring for an App that is probably very difficult or nearly impossible to realistically deliver (see anything to do with discount codes or gift certificates at this time) or the App they are clamouring for is probably coming very soon now to Shopify core, meaning your App will be a tough sell in 3, 6 or a few more months.
If you're looking for low fruit to develop just go for it! There are so many things you could do with the API it's almost best to just make something simple and great and then like the Field of Dreams, they will come.