I like that you are being honest about the fact that you 'think you should launch an app, but don't know what it should be about.' Most brands are in a similar situation and that's why they end up blowing money on endeavours that are never used by customers. My advice would be to think about the core proposition of your brand. Why do your customers buy it? Is there any way to extend that in the digital medium to do something interactive and useful? A really good example is Pantene's collaboration with The Weather Channel, that targets products according to weather, i.e., anti-frizz products shown to women in humid zones. You may not even need an app!