Rework the existing legacy weather app. Make it sense to develop again different apps for android and iOS and what the heck is the different between all the weather apps?
Ask users which weather app they’re using and why. How detailed should be the weather data? How many places there're using? Check the analytic data from existing tools and so on.
A clean and easy to use on-premises app with no frills.
The possibility to get data for different places with weather warning for extreme coldness for example.