To me FPGA technology serves the function to play original-behaving games on a HDMI TV, with an original controller and the optional conveniance to run ROMs.
With a handheld that doesn't require a TV the functionality remains the same always. There's no need to FPGA anything. The games themselves aren't inconvenienced by a flashcart, hell it doesn't even potrude from the console.
If you're into red/blue shining screens, backlit, sound improvements, pimped out casings etc. i guess it can fill a niche. Me i'd rather see the monochrome green GB improved to not have ghosting or the ability to use the GB/GBC as a controller when playing on TV (like the Game Bly Player offered with GBA).