I agree about not turning it into a smart phone and sticking with core functionality, but are you sure that an SD card reader is the right way to do that?
For instance, adding BLE instead of an SD card reader would seem to cover most of the functionality of the SD card reader, though that might depend on exactly what you had in mind for it, as well as allowing multiplayer gaming.
For that matter, I'd much rather have an SD chip and a CPU with the capability to play host. The open source drone tx firmware I work on has three boot modes: production, do the bootloader thing, and "USB drive" for installing model files, etc. AFAIC, that's better than an SD card reader, since I can't lose SD storage I can't remove.