Here, I wanna present you the GX-4000 from Amstrad.
This console was released in September 1990 only in Europe. This is a 8bit console based on the Amstrad CPC architecture, released at a time when 16bit consoles were being released. That resulted in a system that lived barely a year and sold around 150,000 copies.
26 official games has been released and are compatible with Amstrad 464+ and 6128+
The picture above is a rear view and shows 5V is present in the output, so you can connect it to an color Amstrad monitor (sold with 464+ and 6128+) along with the 8-pin DIN plug.
Beware that Amtrad CPC 464 and 6128 A/V connector is different from the 464+, 6128+ and the GX-4000 one!
While the European version of the GX-4000 outputs RGB with Sync-on-Composite (and has a RF modulator), the French version outputs RGB with Clean Sync.
What about Flashcards or Multicarts?
Today, the most complete option is the C4CPC from CPCWiki user Gerald. The price of the PCB + a 8GB µSD card is about 65€.
A 3D Printed case is also available on the page.
The C4CPC can also be used for Amstrad Plus range of Microcomputers.
Beware to not use the OEM power brick but rather a 9V one (see schema above) or you can brick your console!
Here is the thread about it.
A 32-in-1 Multicart can also be found on some site like this one for 35€. I’ve personally never tested it.
This one is only compatible with the GX-4000 due to its size:
Sources: Wikipedia, CPCWiki, Evan Amos’ Wikimedia