SDL on Wii U currently runs at 1280*720 internally, while the Wii U can officially be configured to run at native output resolutions of 854*480, 1280*720 or 1920*1080. All three are fairly valuable, but native 480p in particular is important due to the GamePad. If I've understood the current situation correctly, SDL games designed to run at e.g. 640*480 would currently be forced to run at 640*480 scaled 1.5x to 960*720 scaled back 0.666...x to to 640*480 again for GamePad output.

Instead, SDL should support either running at an internal resolution that is configurable by the developer (which is then scaled to the actual output resolution) or running at the native resolution set on the console. This way, if an app absolutely only works at one resolution, the developer can enforce running at that resolution, otherwise allowing for flexibility. (Internal resolution could also be made an end-user choice if the SDL app's developer wishes to make it configurable, but that's just the developer configuration with extra steps.)

It may be desirable to allow manually configured resolutions to be arbitrary, i.e. not one of the native Wii U resolutions. Taking the example again of a game originally designed to run at 640*480, a non-native resolution like 1708*960 (sub-native for 1080p displays, exactly 2x native 854*480) would allow a 2x nearest-neighbor prescale (1280*960), for a sharper result on the TV than scaling from SDL internal res of 854*480 to 1920*\1080. At the same time, sending the same framebuffer to the GamePad would result in a native 480-line image. This would give nicer results than using any of the native resolutions.

I think that last paragraph can safely be considered a lower priority than just supporting the actual native resolutions of the Wii U. e.g. Running that 640*480 game with a native 480-line display would most likely be adequate for most purposes.