We'd need to solve the problem at the root:
1. Ditch App-Stores and get proper distributions which have strict code control like Debian for example. Yes, this would kill commercial closed source apps, but point 3 will take care of those.
2. Mandate some common hardware platform or a BIOS so the operating system doesn't need to be ported to every device.
3. Design and implement a very simple protocol for "remote GUIs". Essentially this should replace web-apps with something simpler, more secure and faster to use over wireless connections. It would be something like "GUI-toolkit" commands via TLS over Websocket or something. There may be code to implement something like custom GUI elements, where the code can only access the properties of that element and draw in the area of the element as well as get touch information from that area. All the logic would run on the server, everything time critical would run on the client. This is of course trivial to charge for.