A new 3D engine is on the way for the Game Boy Advance called 3DSage that will let you build your own Doom-like levels.
3DSage is the work of the homebrew developer Sage, who has previously produced a bunch of other 3D demos for Nintendo's handheld, including demakes of The Legend of Zelda: Ocarina of Time and GoldenEye 007's multiplayer mode. Recently, he's been posting regular updates about the work-in-progress engine over on YouTube and Twitter and states that it will allow users to create their own Doom-like maps by editing the geometry of levels and replacing textures in the environment.
This can be done either in an emulator or on real hardware, with the developer designing both the controls and the UI to work seamlessly with the limited number of GBA buttons.
The levels can even be edited in 3D, with players being able to aim a reticule at the texture they want to replace and quickly swap it out, in addition to changing the shaders and raising or lowering the floors and ceilings.
All in all, it's a fascinating project and one we can't wait to experiment with upon release. Right now, there doesn't seem to be a precise date set for its launch, but a recent trailer claims it will be "coming soon".