This is a very old and wise saying (by old, I mean the beginning of the tech era/ startup culture) that if you have an idea for a product, first make sure that the product is useful to you, and care about the rest afterwards. Because the best use of a prosuct can only made by a user who needs it and it matches his requirements.
Some ideas of mobile apps that I have in mind, (other than those which I’ve already built apps on:
App based on Near Field Communication
This is a chatting application. This app will use Near Field Communication instead of internet to communicate to nearby users of the same app. The users might be strangers, so the app is a way to talk to strangers.