This Kind of App is already there called as Shutdown7. Now what I like about the app is that its pretty straight forward without any fancy stuff.
There are two suggestions I will give you here, if it can be implemented, to make your app really stand out
1. Timer Function which can shutdown or restart automatically.
2. Option to Shutdown when battery goes low below certain percentage. You can think of grabbing data from battery save mode or Battery App from Singularity.
Otherwise your app is just flawless what people will look for is more features.
