Everyone knows that the GBA was the first TRUE successor to the Game Boy, but Nintendo didn't always have that as the plan. There was originally another piece of hardware in the works that was planned to take over for the Game Boy, but Nintendo shied away from it and instead went with what we now know as the GBA. Check out some details on this unreleased successor below.
– "Project Atlantis" is usually regarded as the GBA's codename, which is NOT true – the GBA was codenamed "Advanced Game Boy"
– "Project Atlantis" was a Game Boy successor that Nintendo developed around 1995. Rumours about the system started in early 1996. It was supposed to be a 32 Bit color handheld, to have four buttons and to have a screen bigger than the final GBA. Media reported the system was supposed to release in late 1996.
– Rumours further suggested that "Project Atlantis" had power comparable to the N64 and used a 160 MHz processor – it would have been way more powerful than the final GBA. Also, "Project Atlantis's" battery time was supposed to be 30 hours.
– Although Nintendo confirmed the system's existence in mid-1996, "Project Atlantis" never got released. The newly released Game Boy Pocket sold well enough so that Nintendo delayed the release of "Project Atlantis" to the end of 1997.
– Finally the system never came to the market. The reason was that the system was simply way too big to be a Nintendo portable system. Also it was too power-consuming, too expensive to manufacture and Nintendo apparently wasn't satisfied with it's performance.
– DSi lead developer Masato Kuwahara who also participated in the development of "Project Atlantis" showed the prototype in a GDC 2009 lecture. Here's the picture he showed, with a DSi as a size comparison: http://nintendo-online.de/upload/images/2016/03/09/110326906656e03cc6097c88-22465637.jpg
– Bonus fact 1: Kuwahara also showed an unreleased Touch Screen Adaptor for the Game Boy Color that he developed in 1998. His picture shows the prototype attached to a Game Boy Advance SP: http://nintendo-online.de/upload/images/2016/03/09/136120641356e03cc6097de3-91806791.jpg
– Bonus fact 2: The dev team also experimented with a fold up model for the Game Boy Advance, the like of which the GBA SP and the DS are. Because back then the system would have been to thick, they discarded the idea.
– Bonus fact 3: The development of the final GBA didn't start until the Game Boy Color was released and only took about two years.