Justin Williams:

My first OS X app was a todo list. My second a calendaring app. My main product on iOS has been a Markdown text editor since 2010. In my defense, the text editor market in 2010 was wide open. Today, I wouldn’t even consider trying to enter the market unless I was completely confident that I was building something that was lightyears ahead of the best product already in the market.

Make whatever app you want. But if you’re making a product in a crowded market, it better be something that stands out from the other apps available.