You can build any kind of app.
Your app could be a general utility app that anybody, anywhere, could use. Task managers, organizers and games, are some examples.
It could also be an app that helps your customers engage more directly and intuitively with your business to consume your products or services.
Apps that help to increase the productivity of your staff, whether they are in the office or on the go, are also great ideas.
As we increasingly spend more time on our smartphones, mobile apps can be a wonderful way to extend your business reach.
The most important thing is that it fits into your overall business strategy and brings value to your business.