Currently when creating an app, there's only 2 options: Tablet & Mobile. However, in the modern world there's so many different types of screen sizes and form factors (desktops, Surface Hub, laptops/tablets of various sizes, phones/phablets, smaller IoT screens, etc.). I think the 'Tablet' and 'Phone' layout options are an outdated concept and should be replaced with 'Responsive Layout', or even better, just remove those options all together and have responsive layout as default for all apps. And instead have various standard screen sizes as 'views' to preview and customize the app for various sizes and have them automatically scale/adjust to those sizes.
Most websites and UWP/PWA apps have already moved in this direction years ago, and it's pretty much standard now to ensure end-users get the best experience regardless of what device they are on.
This would also be great for those those who want to easily transition from InfoPath to PowerApps and not be confused by the current limited layout options.