The Game Boy Advance SP has to rank as one of the best handhelds ever. Not only does it allow you to play GBA titles in an appealing clamshell form factor, but it's also backwards compatible with Game Boy and Game Boy Color software.
Taking all of this into account, you might well question the wisdom of hacking apart a Game Boy Pocket and turning it into a clamshell SP-style device – but modder Allison Parrish has done it anyway (thanks, RetroRGB).
The mod involves slicing the Game Boy Pocket mainboard clean in two and reconnecting the two halves via flexible ribbon cables. The device is placed in a custom-made shell and spruced up with a modern backlit IPS display and rechargeable battery. The lovely people at Macho Nacho Productions have been showing off the console, and it's pretty tasty.
In the end, we've got a device which doesn't really need to exist, given that the GBA SP is already a thing – but that doesn't mean we don't want one of our own (and at least the cartridges don't stick out awkwardly on the Pocket SP, as they do on the GBA).