Does Apple approve widget-only iOS apps?
See the topic I mentioned earlier and search on the page for 'collection of widgets' but here are a few widgets in the App Store which contain multiple widget in one app and work on the Notification Center for iOS8 or higher devices:


Widgets Pro
Wdgts
Widget
Vidgets
Widgets Pro
OneWidget
Widgets.
My Widgets
Widgets - Calculator, Currency and Calendar