....
GX4000 everdrive. ... This likely sold even less than Jaguar. ...
According to Retro Gamer, only 15,000 of these consoles were ever sold. Which, on the plus side, makes it a real collector's item for people who really value rarity (a lot of hardcore collectors do, though I don't see how something being rare automatically makes an item more desirable, myself).
And since (as Retro Gamer said) the console is basically an Amstrad CPC with extra hardware (sprites, scrolling, etc), then if an Everdrive or similar was released for it, it could open up the door for homebrew. Granted, all Everdrives do this for their respective machines anyway, but the CPC is a Z80 based machine, and the Z80 is a very well known and easy to program CPU, being used in lots of different machines, such as the ZX81, ZX Spectrum, Gameboy, Gameboy Colour, Game Gear, Sega Master System, and even some early arcade games. There are tons on finformation about the Z80 on the 'net, and since the CPC was so relatively popular, there are all sorts of programming utilities for it, and people who have progammed on it, making it (and so the GX4000) a much more likely candidate for homebrew than the relatively much less well understood (from the point of view of an amateur coder) consoles that the Evrdrives support.