When you have developed a useful custom community widget, you sure want it to appear on your mobile apps, too.
There seems to be a simple and easy opportunity to achieve this:
If the org extension model for custom community widgets would allow another mode=mobile, admins could easily select the widgets that are capable of being used and properly rendered on a mobile device.
Now, that widget could easily be shown in communty navbar and presented in the inapp browser when selected. Although that may not follow the idea of having the community header on top, it would allow integration of custom applications in an acceptable way to mobile devices without additional dev cost.